Organic photosensitive optoelectronic devices with transparent electrodes

ABSTRACT

An organic photosensitive optoelectronic device optimized to enhance desired characteristics such as external quantum efficiency is described. The photosensitive optoelectronic device has at least two transparent electrodes and one or more organic photoconductive layers disposed between the transparent electrodes. In other embodiments photosensitive optoelectronic devices with multilayer photoconductive structures and photosensitive optoelectronic devices with a reflective layer are disclosed.

FIELD OF INVENTION

[0001] The present invention generally relates to organic thin-filmphotosensitive optoelectronic devices. More specifically, it is directedto organic photosensitive optoelectronic devices, e.g., solar cells andphotodetectors, having transparent electrodes. Further, it is directedto organic photosensitive optoelectronic devices having a transparentcathode which forms a low resistance interface with the adjacent organicsemiconductor layer. Further, it is directed to an organicphotosensitive optoelectronic device consisting of a plurality ofstacked photosensitive optoelectronic subcells, each subcell having oneor more photosensitive optoelectronically active layers and transparentcharge transfer layers.

BACKGROUND OF THE INVENTION

[0002] Optoelectronic devices rely on the optical and electronicproperties of materials to either produce or detect electromagneticradiation electronically or to generate electricity from ambientelectromagnetic radiation. Photosensitive optoelectronic devices convertelectromagnetic radiation into electricity. Solar cells, also known asphotovoltaic (PV) devices, are specifically used to generate electricalpower. PV devices are used to drive power consuming loads to provide,for example, lighting, heating, or to operate electronic equipment suchas computers or remote monitoring or communications equipment. Thesepower generation applications also often involve the charging ofbatteries or other energy storage devices so that equipment operationmay continue when direct illumination from the sun or other ambientlight sources is not available. As used herein the term “resistive load”refers to any power consuming or storing device, equipment or system.Another type of photosensitive optoelectronic device is a photoconductorcell. In this function, signal detection circuitry monitors theresistance of the device to detect changes due to the absorption oflight. Another type of photosensitive optoelectronic device is aphotodetector. In operation a photodetector has a voltage applied and acurrent detecting circuit measures the current generated when thephotodetector is exposed to electromagnetic radiation. A detectingcircuit as described herein is capable of providing a bias voltage to aphotodetector and measuring the electronic response of the photodetectorto ambient electromagnetic radiation. These three classes ofphotosensitive optoelectronic devices may be characterized according towhether a rectifying junction as defined below is present and alsoaccording to whether the device is operated with an external appliedvoltage, also known as a bias or bias voltage. A photoconductor celldoes not have a rectifying junction and is normally operated with abias. A PV device has at least one rectifying junction and is operatedwith no bias. A photodetector has at least one rectifying junction andis usually but not always operated with a bias.

[0003] Traditionally, photosensitive optoelectronic devices have beenconstructed of a number of inorganic semiconductors, e.g. crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride and others. Herein the term “semiconductor” denotes materialswhich can conduct electricity when charge carriers are induced bythermal or electromagnetic excitation. The term “photoconductive”generally relates to the process in which electromagnetic radiant energyis absorbed and thereby converted to excitation energy of electriccharge carriers so that the carriers can conduct, i.e., transport,electric charge in a material. The terms “photoconductor” and“photoconductive material” are used herein to refer to semiconductormaterials which are chosen for their property of absorbingelectromagnetic radiation of selected spectral energies to generateelectric charge carriers. Solar cells are characterized by theefficiency with which they can convert incident solar power to usefulelectric power. Devices utilizing crystalline or amorphous silicondominate commercial applications and some have achieved efficiencies of23% or greater. However, efficient crystalline-based devices, especiallyof large surface area, are difficult and expensive to produce due to theproblems inherent in producing large crystals without significantefficiency-degrading defects. On the other hand, high efficiencyamorphous silicon devices still suffer from problems with stability.More recent efforts have focused on the use of organic photovoltaiccells to achieve acceptable photovoltaic conversion efficiencies witheconomical production costs.

[0004] PV devices typically have the property that when they areconnected across a load and are irradiated by light they produce aphotogenerated voltage. When irradiated without any external electronicload, a PV device generates its maximum possible voltage, Vopen-circuit, or V_(OC). If a PV device is irradiated with itselectrical contacts shorted, a maximum short-circuit current, or I_(SC),is produced. When actually used to generate power, a PV device isconnected to a finite resistive load and the power output is given bythe current voltage product, I×V. The maximum total power generated by aPV device is inherently incapable of exceeding the product,I_(SC)×V_(OC). When the load value is optimized for maximum powerextraction, the current and voltage have values, I_(max) and V_(max)respectively. A figure of merit for solar cells is the fill factor ffdefined as: $\begin{matrix}{{ff} = \frac{I_{\max}V_{\max}}{I_{SC}V_{OC}}} & (1)\end{matrix}$

[0005] where ff is always less than 1 since in actual use I_(SC) andV_(OC) are never obtained simultaneously. Nonetheless, as ff approaches1, the device is more efficient.

[0006] When electromagnetic radiation of an appropriate energy isincident upon a semiconductive organic material, for example, an organicmolecular crystal (OMC) material, or a polymer, a photon can be absorbedto produce an excited molecular state. This is represented symbolicallyas S₀+hv→S₀*. Here S₀ and S₀* denote ground and excited molecularstates, respectively. This energy absorption is associated with thepromotion of an electron from a bound state in the valence band, whichmay be a η-bond, to the conduction band, which may be a π*-bond, orequivalently, the promotion of a hole from the conduction band to thevalence band. In organic thin-film photoconductors, the generatedmolecular state is generally believed to be an exciton, i.e., anelectron-hole pair in a bound state which is transported as aquasi-particle. The excitons can have an appreciable life-time beforegeminate recombination, which refers to the process of the originalelectron and hole recombining with each other as opposed torecombination with holes or electrons from other pairs. To produce aphotocurrent the electron-hole pair must become separated. If thecharges do not separate, they can recombine in a geminate recombinationprocess, either radiatively—re-emitting light of a lower than incidentlight energy—, or non-radiatively—with the production of heat.

[0007] Either of these outcomes is undesirable in a photosensitiveoptoelectronic device. While exciton ionization, or dissociation, is notcompletely understood, it is generally believed to occur in regions ofelectric field occurring at defects, impurities, contacts, interfaces orother inhomogeneities. Frequently, the ionization occurs in the electricfield induced around a crystal defect, denoted, M. This reaction isdenoted S₀*+→e⁻+h⁺. If the ionization occurs at a random defect in aregion of material without an overall electric field, the generatedelectron-hole pair will likely recombine. To achieve a usefulphotocurrent, the electron and hole must be collected separately atrespective opposing electrodes, which are frequently referred to ascontacts. This is achieved with the presence of an electric field in theregion occupied by the carriers. In power generation devices, i.e., PVdevices, this is preferably achieved with the use of internally producedelectric fields that separate the generated photocarriers. In otherphotosensitive optoelectronic devices, the electric field may begenerated by an external bias, e.g., in a photoconductor cell, or as aresult of the superposition of internally and externally generatedelectric fields, e.g., in a photodetector. In an organic PV device, asin other photosensitive optoelectronic devices, it is desirable toseparate as many of the photogenerated electron-hole pairs, or excitons,as possible. The built-in electric field serves to dissociate theexcitons to produce a photocurrent.

[0008]FIG. 1 schematically depicts the photoconductive process inorganic semiconducting materials. Step 101 shows electromagneticradiation incident upon sample of photoconductive material between twoelectrodes a and b. In step 102, a photon is absorbed to generate anexciton, i.e., electron-hole pair, in the bulk. The solid circleschematically represents an electron while the open circle schematicallyrepresents a hole. The curving lines between the hole and electron arean artistic indication that the electron and hole are in an excitonicbound state. In step 103, the exciton diffuses within the bulkphotoconductive material as indicated by the exciton's closer proximityto electrode a. The exciton may suffer recombination in the bulkmaterial away from any field associated with a contact or junction asindicated in step 104. If this occurs the absorbed photon does notcontribute to the photocurrent. Preferably the exciton ionizes withinthe field associated with a contact or junction as indicated by theprogression from step 103 to step 105. However, it is still possible forthe newly liberated carriers to recombine as indicated in step 106before permanently separating and contributing to the photocurrent.Preferably the carriers separate and respond to the field near a contactor junction according to the sign of their electric charge as indicatedby the progression from step 105 to step 107. That is, in an electricfield, indicated by ε in step 107, holes and electrons move in oppositedirections.

[0009] To produce internally generated electric fields which occupy asubstantial volume, the usual method is to juxtapose two layers ofmaterial with appropriately selected conductive properties, especiallywith respect to their distribution of molecular quantum energy states.The interface of these two materials is called a photovoltaicheterojunction. In traditional semiconductor theory, materials forforming PV heterojunctions have been denoted as generally being ofeither n, or donor, type or p, or acceptor, type. Here n-type denotesthat the majority carrier type is the electron. This could be viewed asthe material having many electrons in relatively free energy states. Thep-type denotes that the majority carrier type is the hole. Such materialhas many holes in relatively free energy states. The type of thebackground, i.e., not photogenerated, majority carrier concentrationdepends primarily on unintentional doping by defects or impurities. Thetype and concentration of impurities determine the value of the Fermienergy, or level, within the gap between the highest occupied molecularorbital (HOMO) and the lowest unoccupied molecular orbital (LUMO),called the HOMO—LUMO gap. The Fermi energy characterizes the statisticaloccupation of molecular quantum energy states denoted by the value ofenergy for which the probability of occupation is equal to {fraction(1/2)}. A Fermi energy near the LUMO energy indicates that electrons arethe predominant carrier. A Fermi energy near the HOMO energy indicatesthat holes are the predominant carrier. Accordingly, the Fermi energy isa primary characterizing property of traditional semiconductors and theprototypical PV heterojunction has traditionally been the p-n interface.

[0010] In addition to relative free-carrier concentrations, asignificant property in organic semiconductors is carrier mobility.Mobility measures the ease with which a charge carrier can move througha conducting material in response to an electric field. As opposed tofree carrier concentrations, carrier mobility is determined in largepart by intrinsic properties of the organic material such as crystalsymmetry and periodicity. Appropriate symmetry and periodicity canproduce higher quantum wavefunction overlap of HOMO levels producinghigher hole mobility, or similarly, higher overlap of LUMO levels toproduce higher electron mobility. Moreover, the donor or acceptor natureof an organic semiconductor, e.g., 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA), may be at odds with the higher carrier mobility.For example, while chemistry arguments suggest a donor, or n-type,character for PTCDA, experiments indicate that hole mobilities exceedelectron mobilities by several orders of magnitude so that the holemobility is a critical factor. The result is that device configurationpredictions from donor/acceptor criteria may not be borne out by actualdevice performance. Thus, in selecting organic materials such as thosedescribed herein for photosensitive optoelectronic devices, it has beenfound that isotype heterojunctions, e.g, p-p, may have rectifyingproperties as good as traditional p-n type heterojunctions, althoughtrue p-n type is generally preferable when possible. Isotypeheterojunctions are discussed further below. Due to these uniqueelectronic properties of organic materials, rather than designating themas “p-type” or “n-type”, the nomenclature of “hole-transporting-layer”(HTL) or “electron-transporting-layer” (ETL) is frequently used. In thisdesignation scheme, an ETL will preferentially be electron conductingand an HTL will preferentially be hole transporting. The term“rectifying” denotes, inter alia, that an interface has an asymmetricconduction characteristic, i.e., the interface supports electroniccharge transport preferably in one direction. Rectification isassociated normally with a built-in electric field which occurs at theheterojunction between appropriately selected materials.

[0011] The electrodes, or contacts, used in a photosensitiveoptoelectronic device are an important consideration. In aphotosensitive optoelectronic device, it is desirable to allow themaximum amount of ambient electromagnetic radiation from the deviceexterior to be admitted to the photoconductively active interior region.That is, it is desirable to get the electromagnetic radiation to whereit can be converted to electricity by photoconductive absorption. Thisindicates that at least one of the electrical contacts should beminimally absorbing and minimally reflecting of the incidentelectromagnetic radiation. That is, the contact should be substantiallytransparent. When used herein, the terms “electrode” and “contact” referonly to layers that provide a medium for delivering photogenerated powerto an external circuit or providing a bias voltage to the device. Thatis, an electrode, or contact, provides the interface between thephotoconductively active regions of an organic photosensitiveoptoelectronic device and a wire, lead, trace or other means fortransporting the charge carriers to or from the external circuit. Theterm “charge transfer layer” is used herein to refer to layers similarto but different from electrodes in that a charge transfer layer onlydelivers charge carriers from one subsection of an optoelectronic deviceto the adjacent subsection. As used herein, a layer of material or asequence of several layers of different materials is said to be“transparent” when the layer or layers permit at least 50% of theambient electromagnetic radiation in relevant wavelengths to betransmitted through the layer or layers.

[0012] When an electrode or charge transfer layer provides the primarymechanism for photovoltaic charge separation, the device is called aSchottky device as discussed further below.

[0013] Electrodes or contacts are usually metals or “metal substitutes”.Herein the term “metal” is used to embrace both materials composed of anelementally pure metal, e.g., Mg, and also metal alloys which arematerials composed of two or more elementally pure metals, e.g., Mg andAg together, denoted Mg:Ag. Here, the term “metal substitute” refers toa material that is not a metal within the normal definition, but whichhas the metal-like properties that are desired in certain appropriateapplications. Commonly used metal substitutes for electrodes and chargetransfer layers would include wide bandgap semiconductors, for example,transparent conducting oxides such as indium tin oxide (ITO), tin oxide(TO), gallium indium tin oxide (GITO), zinc oxide (ZO) and zinc indiumtin oxide (ZITO). In particular, ITO is a highly doped degenerate n+semiconductor with an optical bandgap of approximately 3.2 eV renderingit transparent to wavelengths greater than approximately 3900 Å. Anothersuitable metal substitute material is the transparent conductive polymerpolyanaline (PANI) and its chemical relatives. Metal substitutes may befurther selected from a wide range of non-metallic materials, whereinthe term “non-metallic” is meant to embrace a wide range of materialsprovided that the material is free of metal in its chemically uncombinedform. When a metal is present in its chemically uncombined form, eitheralone or in combination with one or more other metals as an alloy, themetal may alternatively be referred to as being present in its metallicform or as being a “free metal”. Thus, the metal substitute electrodesof the present invention may sometimes be referred to by one or more ofthe inventors of the present invention as “metal-free” wherein the term“metal-free” is expressly meant to embrace a material free of metal inits chemically uncombined form. Free metals typically have a form ofmetallic bonding that may be thought of as a type of chemical bondingthat results from a sea of valence electrons which are free to move inan electronic conduction band throughout the metal lattice. While metalsubstitutes may contain metal constituents they are “non-metallic” onseveral bases. They are not pure free-metals nor are they alloys offree-metals. Further, these metal substitutes do not have their Fermilevel in a band of conducting states in contrast with true metals. Whenmetals are present in their metallic form, the electronic conductionband tends to provide, among other metallic properties, a highelectrical conductivity as well as a high reflectivity for opticalradiation. Another characteristic of metallic conductors is thetemperature dependence of their conductivity. Metals generally have ahigh conductivity at room temperature which increases as the temperatureis lowered to near absolute zero. Metal substitutes, for example,semiconductors including, inter alia, inorganic, organic, amorphous, orcrystalline, generally have conductivities which decrease as theirtemperature is lowered to near absolute zero.

[0014] There are two basic organic photovoltaic device configurations.The first type is the Schottky-type cell with a single species oforganic photoconductive material sandwiched between a pair of metaland/or metal substitute contacts. Conventionally, for n-typephotoconductors, a high work function metal, e.g., Au, has been used asthe Schottky contact, and for p-type photoconductors, a metal with a lowwork function, e.g., Al, Mg, or In has been used as the Schottkycontact. The charge separation desired in a PV device is induced byexciton dissociation in the space-charge region associated with thebuilt-in electric field at the metal/photoconductor interface.Conventionally, such a device requires different metal or metalsubstitute pair combinations as contacts since use of the same materialat both interfaces would ostensibly produce opposing rectifyingjunctions. If the same material is used for both electrodes it has beengenerally thought that the fields generated at thephotoconductor-electrode interfaces are necessarily equal in magnitudeand opposite in direction so that no net photocurrent is generated inthe absence of an external applied voltage. While it is possible forcharge separation to occur a. both interfaces and be additive, it isgenerally preferable to have all charge separation occurring at oneinterface. For example, this can be achieved if the non-rectifyinginterface has little or no barrier to carrier transport, i.e., if it isa relatively low resistance contact. This may also be referred to as an“ohmic” contact. In any event, in photosensitive optoelectronic devicesit is generally desirable that the interfaces either contribute to thenet charge separating action or present the smallest possible resistanceor barrier to carrier transport.

[0015] A sample prior art organic Schottky device is shown schematicallyin FIG. 2A. Contact 2A01 is Ag; organic photoconductive layer 2A02 isPTCDA; and contact 2A03 is ITO. Such a cell was described in an articleby N. Karl, A. Bauer, J. Holzäofel, J. Marktanner, M. Möbus, and F.Stölzle, “Efficient Organic Photovoltaic Cells: The Role of ExcitonicLight Collection, Exciton Diffusion to Interfaces, Internal Fields forCharge Separation, and High Charge Carrier Mobilities”, MolecularCrystals and Liquid Crystals,Vol. 252, pp 243-258, 1994 (hereinafterKarl et al.). Karl et al. noted that while the Ag electrode 2A01 wasphotovoltaically active in such a cell, the ITO electrode very rarelywas photoactive and further that the indications of photovoltaic actionat the ITO electrode had poor statistical certainty. Further, one ofordinary skill in the art would expect contact 2A01 not to betransparent.

[0016] The second type of photovoltaic device configuration is theorganic bilayer cell. In the bilayer cell, charge separationpredominantly occurs at the organic heterojunction. The built-inpotential is determined by the HOMO—LUMO gap energy difference betweenthe two materials contacting to form the heterojunction. An isotypeheterojunction has been discussed in an article by S. R. Forrest, L. Y.Leu, F. F. So, and W. Y. Yoon, “Optical and Electrical Properties ofIsotype Crystalline Molecular Organic Heterojunctions ”, Journal ofApplied Physics,Vol. 66, No. 12, 1989 (hereinafter “Forrest, Leu etal.”) and in an article by Forrest, S. R., “Ultrathin Organic FilmsGrown by Organic Molecular Beam Deposition and Related Techniques”,Chemical Reviews, Vol. 97, No. 6, 1997 (hereinafter Forrest, Chem. Rev.1997) both of which are incorporated herein by reference. Forrest, Leuet al. describe two isotype solar cells depicted in FIG. 2B and FIG. 2C.FIG. 2B shows a device consisting of an ITO electrode 2B02 on asubstrate 2B01 covered with a layer 2B03 of copper phthalocyanine (CuPc)and a layer 2B04 of PTCDA with a top electrode 2B05 of In. In a seconddevice, with reference to FIG. 2C, an ITO electrode 2C02 is again placedon a substrate 2C01. Then a CuPc layer 2C03 and a3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) layer 2C04are placed in order with a Ag electrode 2C05 on top. This prior art hadonly one transparent electrode and it was on the bottom of the device.It was also noted in this reference that these organic photovoltaicdevices suffered from a high series resistance.

[0017] As in the case of Schottky devices, unless an interface, at acontact, for example, is contributing to the charge separation, it isdesirable that the interface produce the smallest possible obstructionto free current flow. In bilayer devices, since the dominant chargeseparating region is located near the heterojunction, it is desirablefor the interfaces at the electrodes to have the smallest possibleresistance. In particular, it is known in the art to use thin metallayers as low resistance, or ohmic, electrodes, or contacts. When ohmiccontacts are desired, a high work function metal, e.g., Au, has beenused as the positive electrode, i.e., anode, in photosensitiveoptoelectronic devices. Similarly, a low work function metal, e.g., Al,Mg, or In, has been used to make an ohmic negative electrode, i.e.,cathode.

[0018] Herein, the term “cathode” is used in the following manner. In aPV device under ambient irradiation and connected with a resistive loadand with no externally applied voltage, e.g., a solar cell, electronsmove to the cathode from the adjacent photoconducting material. With anapplied bias voltage, electrons may move from the cathode to theadjacent photoconducting material, or vice versa, depending on thedirection and magnitude of the applied voltage. For example, under“forward-bias” a negative bias is applied to the cathode. If themagnitude of the forward-bias equals that of the internally generatedpotential there will be no net current through the device. If theforward-bias potential exceeds the internal potential in magnitude therewill be a current in the opposite direction from the non-biasedsituation. In this later forward-bias situation, electrons move from thecathode into the adjacent photoconductive organic layer. Under“reverse-bias”, a positive bias is applied to the cathode and anyelectrons which can move do so in the same direction as in the no biassituation. A reverse-biased device generally has little or no currentflow until it is irradiated. Similarly, the term “anode” is used hereinsuch that in a solar cell under illumination, holes move to the anodefrom the adjacent photoconducting material, which is equivalent toelectrons moving in the opposite manner. The application of an externalvoltage to the device structure will alter the flow of the carriers atthe anode/photoconductor interface in a complementary fashion to thatdescribed for the cathode and in a manner understood by those ofordinary skill in the art. It will be noted that as the terms are usedherein anodes and cathodes may be electrodes or charge transfer layers.

[0019] Further, as discussed above, in non-Schottky photosensitiveoptoelectronic devices it is similarly desirable for the electrodes notmerely to form ohmic contacts but also to have high opticaltransparency. Transparency requires both low reflectivity and lowabsorption. Metals have the desired low resistance contact properties;however, they can produce significant conversion efficiency reductionsdue to reflection of ambient radiation away from the device. Also, metalelectrodes can absorb significant amounts of electromagnetic radiation,especially in thick layers. Therefore, it has been desirable to find lowresistance, high transparency electrode materials and structures. Inparticular, the metal substitute ITO has the desired optical properties.It is also known in the art that ITO functions well as an anode inorganic optoelectronic devices. However, it had not been previouslythought that ITO or other metal substitutes could make low resistancecathodes for organic optoelectronic devices. Solar cells had beendisclosed in which a highly transparent ITO layer may have functioned asa cathode in some cases, but such ITO cathodes were only disclosed ashaving been prepared by depositing the charge-carrying organic layeronto the ITO layer by Karl et al. and Whitlock, J. B., Panayotatos, P.,Sharma, G. D., Cox, M. D., Savers, R. R., and Bird, G. R.,“Investigations of Materials and Device Structures for OrganicSemiconductor Solar Cells,” Optical Eng., Vol. 32, No. 8, 1921-1934(August 1993), (Whitlock et al).

[0020] Prior art PV devices, e.g., FIG. 2A and 2B, have only utilizednon-metallic materials, e.g., indium tin oxide (ITO), as one electrodeof the photovoltaic device. The other electrode has traditionally been anon-transparent metallic layer, e.g., aluminum, indium, gold, tin,silver, magnesium, lithium, etc. or their alloys, selected on the basisof work function as discussed above. U.S. Pat. No. 5,703,436 to Forrest,S. R. et al. (hereinafter Forrest ‘436), incorporated herein byreference, describes a technique for fabricating organic photoemissivedevices (TOLEDs: Transparent Organic Light Emitting Diodes) having atransparent cathode deposited onto an organic ETL by depositing a thinmetallic layer, e.g., Mg:Ag, onto the organic ETL and then sputterdepositing an ITO layer onto the Mg:Ag layer. Such a cathode having theITO layer sputter deposited onto a Mg:Ag layer is referred to herein asa “composite ITO/Mg:Ag cathode”. The composite ITO/Mg:Ag cathode hashigh transmission as well as low resistance properties.

[0021] It is known in the art of inorganic solar cells to stack multiplephotovoltaic cells to create an inorganic multisection solar cell withtransparent metallic layers. For example, U.S. Pat. No. 4,255,211 toFrass (hereinafter “Frass ‘211”) discloses a stacked cell arrangement.However, the photolithographic techniques used to fabricate inorganicelectronic devices are typically inapplicable to production of organicoptoelectronic devices . Photolithography generally involves depositionof metallic layers and inorganic semiconductive layers followed byadditional steps of masking and etching. The etching steps involve useof strong solvents which can dissolve the relatively fragile organicsemiconductor materials that are suitable for organic photovoltaicdevices. Therefore, organic photosensitive optoelectronic devicefabrication techniques typically avoid this type of liquid etchingprocess in which deposited material is removed after an organic layerhas been deposited. Instead, device layers are generally depositedsequentially with techniques such as evaporation or sputtering. Accessto electrodes is generally implemented using masking or dry etchingduring deposition. This constraint presents a challenge to fabricationof a stacked organic optoelectronic device for which electrode access tothe intervening layers in the stack is desired. Thus, it is believedthat all prior art stacked cells have the individual photovoltaic cellselectrically connected internally and only in series.

[0022] For inorganic photovoltaic devices, series connection is notparticularly disadvantageous. However, due to the high series resistanceof the organic photovoltaic devices noted above, a series configurationis undesirable for power applications due to the reduced efficiency.Forrest, Chem. Rev. 1997 reported that high series resistance in organicsolar cells leads to space-charge build-up as power levels are raisedwith increasing incident light intensity. This leads to degradation ofthe photocurrent, I_(max), effectively reducing the fill factor andtherefore the efficiency. Moreover, what is believed to be the onlypreviously disclosed organic solar cell with more than one photovoltaicsubcell was a tandem, i.e., two PV subcells, with the subcells connectedin series. See Effect of Thin Gold Interstitial-layer on thePhotovoltaic Properties of Tandem Organic Solar Cell, Hiramoto, M.;Suezaki, M; Yokoyama, M; Chemistry Letters 1990, 327 (hereinafter“Hiramoto”). Referring to FIG. 2D, substrate 2D01 is glass; 2D02 is ITO;2D03 is Me-PTC (500 Å); 2D04 is H₂Pc (700 Å); 2D05 is Au (<30 Å); 2D06is Me-PTC (700 Å); H₂Pc (700 Å); and 2D07 is Au (200 Å). This device hasthe subcells electrically connected internally and in series, thusavoiding the problem of devising a means to make external contact to anelectrode within the middle of a stack of organic semiconductingmaterial. Hiramoto's organic tandem devices have just two electrodes:one on top and bottom used to make external connections plus chargetransfer layer 2D05 which electrically “floats” between the twosubcells”. Only one of the electrodes, bottom ITO layer 2D02 wastransparent. Top Au layer 2D07 was 200 Å thick and thereforenon-transparent. Further, for the reasons noted above, series connectionis not an optimal configuration in stacked organic photovoltaic devicesfor high power applications.

[0023] A solar cell may be viewed as a photodiode with no applied bias.The internal electric field generates a photocurrent when light isincident on the solar cell and the current drives a resistive load forthe extraction of power. On the other hand, a photodetector may beviewed as a diode with no externally applied bias voltage or a finiteexternally applied bias voltage. When electromagnetic radiation isincident upon a photodetector with a bias, the current increases fromits dark value to a value proportional to the number of photogeneratedcarriers and the increase may be measured with external circuitry. If aphotodiode is operated with no applied bias, an external circuit may beused to measure the photogenerated voltage and achieve photodetection.While the same general configuration of electrodes, charge transferlayers and photoconductive layers may be used alternatively as a solarcell or as a photodetector, a configuration optimized for one purpose isgenerally not optimal for another. For example, photosensitiveoptoelectronic devices produced as solar cells are designed to convertas much of the available solar spectrum as possible to electricity.Therefore, a broad spectral response over the entire visible spectrum isdesirable. On the other hand, a photodetector may be desired which has aphotosensitive response over a narrow spectral range or over a rangeoutside the visible spectrum.

[0024] Organic PV devices typically have relatively low quantum yield(the ratio of photons absorbed to carrier pairs generated, orelectromagnetic radiation to electricity conversion efficiency), beingon the order of 1 % or less. This is in part thought to be due to thesecond order nature of the intrinsic photoconductive process, that is,carrier generation requires exciton generation, diffusion andionization, as described above. In order to increase these yields,materials and device configurations are desirable which can enhance thequantum yield and, therefore, the power conversion efficiency.

[0025] Forrest Chem. Rev. 1997 and Arbour, C., Armstrong, N. R., Brina,R., Collins, G., Danziger, J.-P., Lee, P., Nebesny, K. W., Pankow, J.,Waite, S., “Surface Chemistries and Photoelectrochemistries of Thin FilmMolecular Semiconductor Materials” Molecular Crystals and LiquidCrystals, 1990, 183, 307, (hereinafter Arbour et al.), incorporatedherein by reference in its entirety, disclose that alternating thinmultilayer stacks of similar type photoconductors could be used toenhance photogenerated carrier collection efficiency over that using asingle layer structure. Further, these sources describe multiple quantumwell (MQW) structures in which quantum size effects occur when the layerthicknesses become comparable to the exciton dimensions.

SUMMARY AND OBJECTS OF INVENTION

[0026] The present invention is directed to organic photosensitiveoptoelectronic devices utilizing transparent electrodes, in particular,devices that include an organic photosensitive optoelectronic cellcomprised of at least one pair of two transparent electrodes, that is, atransparent cathode and a transparent anode, or devices that have atransparent electrode in superposed relationship upon the top surface ofa substrate with at least one photoconductive organic layer disposedbetween the electrode and the substrate. More specifically, the organicphotosensitive optoelectronic devices of the present invention may becomprised of a transparent cathode that is highly transparent and/orhighly efficient. As representative embodiments, such transparentcathodes may be the highly transparent, highly efficient and/or lowresistance non-metallic or metallic/non-metallic composite cathodes,such as disclosed in the co-pending applications Ser. Nos. 08/964,863and 09/054,707 or in Forrest ‘436, each of which being incorporated inits entirety by reference.

[0027] The organic photosensitive optoelectronic devices of the presentinvention may function as a solar cell, photodetector or photocell.Whenever the organic photosensitive optoelectronic devices of thepresent invention function as solar cells, the materials used in thephotoconductive organic layer or layers and the thicknesses thereof maybe selected, for example, to optimize the external quantum efficiency ofthe device. Whenever the organic photosensitive optoelectronic devicesof the present invention function as photodetectors or photocells, thematerials used in the photoconductive organic layer or layers and thethicknesses thereof may be selected, for example, to maximize thesensitivity of the device to desired spectral regions. In each case, useof transparent electrodes, or even only a transparent top electrode,makes it possible for substantially higher external quantum efficienciesand/or photosensitivities in selected spectral regions to be realizedcompared to when one or more of the electrodes can cause substantialtransmission losses due to absorption and/or reflection losses.

[0028] In addition to the organic photosensitive optoelectronic devicesthat may be comprised of two transparent electrodes or a transparent topelectrode, the present invention is further directed to organicphotosensitive optoelectronic devices having the unique geometric andelectrical configurations that may be fabricated using stacked cellswith transparent electrodes. In particular, the organic photosensitiveoptoelectronic device may be a stacked device comprised of a pluralityof subcells in superposed relation to each other on the surface of asubstrate. The materials and thicknesses of the individual subcells maybe selected, for example, together with selecting the total number ofsubcells that are included in the stacked photosensitive optoelectronicdevice, so as to optimize the external quantum efficiency of thephotosensitive optoelectronic device.

[0029] In particular, for stacked photosensitive optoelectronic devicesconfigured to be electrically connected in parallel, the thicknesses ofthe individual subcells may be adjusted so that in combination withselecting the total number of subcells in the stacked device, theexternal quantum efficiency of the device may be optimized so as toobtain an external quantum efficiency that is higher than that which ispossible for a single cell. The term “external quantum efficiency” isused herein to refer to the efficiency with which a photosensitiveoptoelectronic device is capable of converting the total incidentradiation into electrical power, as distinct from the term “internalquantum efficiency,” which is used herein to refer to the efficiencywith which a photosensitive optoelectronic device is capable ofconverting the absorbed radiation into electrical power. Using theseterms, a stacked photosensitive optoelectronic device with anelectrically parallel configuration may be designed to achieve anexternal quantum efficiency, under a given set of ambient radiationconditions, that approaches the maximum internal quantum efficiency thatmay be achieved for an individual subcell under such ambient conditions.

[0030] This result may be achieved by considering several guidelinesthat may be used in the selection of layer thicknesses. It is desirablefor the exciton diffusion length, L_(D), to be greater than orcomparable to the layer thickness, L, since it is believed that mostexciton dissociation will occur at an interface. If L_(D) is less thanL, then many excitons may recombine before dissociation. It is furtherdesirable for the total photoconductive layer thickness to be of theorder of the electromagnetic radiation absorption length, 1/α (where ais the absorption coefficient), so that nearly all of the radiationincident on the solar cell is absorbed to produce excitons. However, thelayer thicknesses should not be so large compared to the extent of theheterojunction electric fields that many excitons get generated in afield-free region. One reason for this is that the fields help todissociate the excitons. Another reason is that if an excitondissociates in a field-free region, it is more likely to suffer geminantrecombination and contribute nothing to the photocurrent. Furthermore,the photoconductive layer thickness should be as thin as possible toavoid excess series resistance due to the high bulk resistivity oforganic semiconductors.

[0031] Accordingly, these competing guidelines inherently requiretradeoffs to be made in selecting the thickness of the photoconductiveorganic layers of a photosensitive optoelectronic cell. Thus, on the onehand, a thickness that is comparable or larger than the absorptionlength is desirable in order to absorb the maximum amount of incidentradiation. On the other hand, as the photoconductive layer thicknessincreases, two undesirable effects are increased. One is that due to thehigh series resistance of organic semiconductors, an increased organiclayer thickness increases device resistance and reduces efficiency.Another undesirable effect is that increasing the photoconductive layerthickness increases the likelihood that excitons will be generated farfrom the effective field at a charge-separating interface, resulting inenhanced probability of geminate recombination and, again, reducedefficiency. Therefore, a device configuration is desirable whichbalances between these competing effects in a manner that produces ahigh quantum efficiency for the overall device.

[0032] In particular, by taking the above-noted competing effects intoaccount, that is, the absorption length of the photoconductive materialsin the device, the diffusion length of the excitons in these materials,the photocurrent generation efficiency of these excitons, and theresistivity of these materials, the thickness of the layers in anindividual cell may be adjusted so as to obtain a maximum internalquantum efficiency for those particular materials for a given set ofambient radiation conditions. Since the diffusion length of the excitonstends to have a relatively small value and the resistivity of typicalphotoconductive materials tends to be relatively large, an optimalsubcell with respect to achieving the maximum internal quantumefficiency would typically be a relatively thin device. However, sincethe absorption length for such photoconductive organic materials tendsto be relatively large as compared with the exciton diffusion length,such thin optimal photosensitive optoelectronic subcells, which may havethe maximum internal quantum efficiency, would tend to have a relativelylow external quantum efficiency, since only a small fraction of theincident radiation would be absorbed by such optimal subcells.

[0033] So as to improve the external quantum efficiency of an individualsubcell, the thickness of the photoconductive organic layers may beincreased so as to absorb significantly more incident radiation.Although the internal quantum efficiency for converting the additionallyabsorbed radiation into electrical power might gradually decrease as thethickness is increased beyond its optimal subcell thickness, theexternal quantum efficiency of the subcell would still increase until acertain thickness is reached where no further increase in absorptioncould produce an increase in external quantum efficiency. Since theinternal quantum efficiency of the subcell tends to drop rather sharplyas the thickness of the photoconductive layers increases much beyond thediffusion length of the photogenerated excitons, the maximum externalquantum efficiency of the subcell may be achieved well before thethickness of the thicker subcell is sufficient to absorb substantiallyall the incident radiation. Thus, the maximum external quantumefficiency that may be achieved using this single, thicker-cell approachis limited not only by the fact that the subcell thickness may besignificantly greater than that desired for achieving the maximuminternal quantum efficiency but, in addition, such thicker subcells maystill not absorb all the incident radiation. Thus, due to both of theseeffects, the maximum external quantum efficiency of the thicker subcellwould be expected to be significantly less than the maximum internalquantum efficiency that can be achieved for an optimal subcell havingthe optimal thickness.

[0034] A particular feature of the present invention having the stackedorganic photosensitive optoelectronic device with the electricallyparallel configuration is that instead of attempting to improve theexternal quantum efficiency by increasing the thickness of a singlesubcell, which sacrifices the internal quantum efficiency, subcells thathave a thickness that is optimal or near optimal for achieving themaximum internal quantum efficiency may be used to fabricate a stackedstructure. The total number of such optimal subcells that are includedin the stacked structure may be increased so as to provide an increasein absorption of the incident radiation with the total number beinglimited by that which produces no further increase in the externalquantum efficiency. The net result of this approach for improving theexternal quantum efficiency is that a stacked organic photosensitiveoptoelectronic device can be made to have an external quantum efficiencyapproaching the maximum value of the internal quantum efficiency thatcan be achieved for an individual optimal subcell. The improved externalquantum efficiency of the stacked devices may be attributed in largepart to the fact that the subcells of the stacked device may becomprised of pairs of transparent electrodes and, in some cases, also ofa transparent top electrode.

[0035] Taking into account that the additional subcells of the stackeddevice tend to introduce additional losses, such as that due to theresidual reflectivity of the transparent electrodes, the maximumexternal quantum efficiency that can be achieved for a fully optimizedstacked device would typically be somewhat less than the internalquantum efficiency of an optimal subcell. Nevertheless, using themethods of the present invention for optimizing the external quantumefficiency of an organic photosensitive optoelectronic device,substantially higher external quantum efficiencies may be achieved for astacked device than are possible for a device having a single cell,which is optimized for external qunatum efficiency at the expense ofinternal quantum efficiency.

[0036] Since the organic photosensitive optoelectronic devices of thepresent invention may be desired for widely varying ambient radiationconditions, for example, with respect to the intensity of incidentradiation and/or with respect to the spectral distribution of theincident radiation, the photoconductive organic materials, and the layerthicknesses thereof, may be selected so as to be optimized for a givenset of ambient conditions. For example, the photoconductive organicmaterials may be selected to have absorption maxima in selected spectralregions. Since the photoconductive organic materials that may be used ina photosensitive optoelectronic cell may typically have absorptionmaxima only over a limited spectral range, it is an additional featureof the present invention that the stacked photosensitive optoelectronicdevices may be comprised of different types of cells havingphotoconductive organic materials with different absorptioncharacteristics so as to more effectively utilize the entire spectralrange of the incident radiation.

[0037] When the term “subcell” is used hereafter, it may refer to aorganic photosensitive optoelectronic construction of the unilayer,bilayer or multilayer type. When a subcell is used individually as aphotosensitive optoelectronic device, it typically includes a completeset of electrodes, i.e., positive and negative. As disclosed herein, insome stacked configurations it is possible for adjacent subcells toutilize common, i.e., shared, electrode or charge transfer layers. Inother cases, adjacent subcells do not share common electrodes or chargetransfer layers. The term “subcell” is disclosed herein to encompass thesubunit construction regardless of whether each subunit has its owndistinct electrodes or shares electrodes or charge transfer layers withadjacent subunits. Herein the terms “cell”, “subcell”, “unit”,“subunit”, “section”, and “subsection” are used interchangeably to refera photoconductive layer or set of layers and the adjoining electrodes orcharge transfer layers. As used herein, the terms “stack”, “stacked”,“multisection” and “multicell” refer to any optoelectronic device withmultiple layers of a photoconductive material separated by one or moreelectrode or charge transfer layers.

[0038] Since the stacked subcells of the solar cell may be fabricatedusing vacuum deposition techniques that allow external electricalconnections to be made to the electrodes separating the subcells, eachof the subcells in the device may be electrically connected either inparallel or in series, depending on whether the power and/or voltagegenerated by the solar cell is to be maximized. The improved externalquantum efficiency that may be achieved for the stacked solar cells ofthe present invention may also be attributed to the fact that thesubcells of the stacked solar cell may be electrically connected inparallel since a parallel electrical configuration permits substantiallyhigher fill factors to be realized than when the subcells are connectedin series. It is believed that this parallel electrical configuration ofthe stacked subcells is a further unique aspect of the presentinvention.

[0039] Although the high series resistance of photoconductive organicmaterials inhibits use of subcells in a series configuration for highpower applications, there are certain applications, for example, inoperating liquid crystal displays (LCD), for which a higher voltage maybe required, but only at low current and, thus, at low power levels. Forthis type of application, stacked, series-connected solar cells may besuitable for providing the required voltage to the LCD. In the case whenthe solar cell is comprised of subcells electrically connected in seriesso as to produce such a higher voltage device, the stacked solar cellmay be fabricated so as to have each subcell producing approximately thesame current so to reduce inefficiency. For example, if the incidentradiation passes through in only one direction, the stacked subcells mayhave an increasing thickness with the outermost subcell, which is mostdirectly exposed to the incident radiation, being the thinnest.Alternatively, if the subcells are superposed on a reflective surface,the thicknesses of the individual subcells may be adjusted to accountfor the total combined radiation admitted to each subcell from theoriginal and reflected directions.

[0040] Further, it may be desirable to have a direct current powersupply capable of producing a number of different voltages. For thisapplication, external connections to intervening electrodes could havegreat utility and are not believed to have been previously disclosed.Accordingly, in addition to being capable of providing the maximumvoltage that is generated across the entire set of subcells, the stackedsolar cells of the present invention may also be used to providemultiple voltages from a single power source by tapping a selectedvoltage from a selected subset of subcells.

[0041] The present invention may be further described as being directedtoward a method of fabricating photosensitive optoelectronic devicescomprising fabricating a first photosensitive optoelectronic subcell ona substrate so as to form a photosensitive optoelectronic cell capableof producing a given external quantum efficiency, and fabricating asecond photosensitive optoelectronic subcell in superposed relationshipupon the top surface of the first photosensitive optoelectronic subcellso as to form a stacked photosensitive optoelectronic device so as toincrease the external quantum efficiency capability of thephotosensitive optoelectronic cell, wherein at least one of the subcellsof the stacked photosensitive optoelectronic cell is comprised of a pairof transparent electrodes.

[0042] The present invention may be further described as being directedtoward a method of fabricating a series stacked organic photosensitiveoptoelectronic device comprising fabricating a first organicphotosensitive optoelectronic subcell on a substrate so as to form anorganic photosensitive optoelectronic device capable of producing agiven voltage, and fabricating a second organic photosensitiveoptoelectronic subcell in superposed relationship upon the top surfaceof the first organic photosensitive optoelectronic subcell so as to forma stacked organic photosensitive optoelectronic device and so as toincrease the voltage capability of the organic photosensitiveoptoelectronic device, wherein the subcells of the stacked organicphotosensitive optoelectronic cell are comprised of a pair oftransparent electrode layers and the first subcell and the secondsubcell are electrically connected in series.

[0043] The present invention may be further described as being directedtoward a method of fabricating a parallel stacked organic photosensitiveoptoelectronic device comprising fabricating a first organicphotosensitive optoelectronic subcell on a substrate so as to form anorganic photosensitive optoelectronic device capable of producing agiven external quantum efficiency, and fabricating a second organicphotosensitive optoelectronic subcell in superposed relationship uponthe top surface of the first organic photosensitive optoelectronicsubcell so as to form a stacked organic photosensitive optoelectronicdevice so that the external quantum efficiency capability of the organicphotosensitive optoelectronic device is increased, wherein the firstsubcell and the second subcell are electrically connected in parallel.

[0044] The present invention may be further described as being directedtoward a mixed electrical configuration stacked organic photosensitiveoptoelectronic device comprising a substrate having a proximal surfaceand a distal surface, and a plurality of subassemblies of organicphotosensitive optoelectronic subcells, each of the subcells having acathode and an anode, each of the cathode and anode being an electrodelayer or a charge transfer layer, the subcells in superposed relationwith each other and with the distal surface of the substrate, each ofthe subassemblies of subcells comprising a plurality of subcellselectrically connected in parallel or a plurality of subcellselectrically connected in series, wherein the subassemblies areelectrically connected to each other in series or in parallel such thatthe device includes subcells electrically arranged in series andparallel, so that the device is capable of producing a voltage higherthan possible with a completely parallel arrangement with the samematerials and with higher external quantum efficiency than a completelyseries arrangement for producing the same voltage.

[0045] The present invention may be further described as being directedtoward a method of fabricating a mixed electrical configuration stackedorganic photosensitive optoelectronic device comprising: fabricating afirst organic photosensitive optoelectronic subcell on a substrate so asto form an organic photosensitive optoelectronic device; fabricating asecond organic photosensitive optoelectronic subcell in superposedrelationship upon the top surface of the first organic photosensitiveoptoelectronic subcell so as to form a first stacked organicphotosensitive optoelectronic subassembly comprised of the first subcelland the second subcell electrically connected in series; fabricating athird organic photosensitive optoelectronic subcell in superposedrelationship upon the top surface of the second organic photosensitiveoptoelectronic subcell; and fabricating a fourth organic photosensitiveoptoelectronic subcell in superposed relationship upon the top surfaceof the third organic photosensitive optoelectronic subcell so as to forma second stacked organic photosensitive optoelectronic subassemblycomprising the third subcell and the fourth subcell electricallyconnected in series, wherein the first stacked organic photosensitiveoptoelectronic subassembly and the second stacked organic photosensitiveoptoelectronic subassembly are electrically connected in parallel.

[0046] Representative embodiments may also comprise transparent chargetransfer layers. As described herein charge transfer layers aredistinguished from ETL and HTL layers by the fact that charge transferlayers are frequently, but not necessarily, inorganic and they aregenerally chosen not to be photoconductively active. That is, theelectrodes and charge transfer layers preferably do not absorbelectromagnetic radiation for conversion to electrical or thermal formsof energy. Therefore, transparent low reflectivity electrodes and chargetransfer layers are generally preferred in the present invention. Inaddition, the electrode and charge transfer layer electronic propertiesare important. In certain device configurations one or more of theelectrodes or charge transfer layers may be electronically active. Forexample, as discussed above, an electrode or charge transfer layer mayprovide an interfacial region for dissociating or recombining excitons,or it may provide a rectifying interface. In other deviceconfigurations, it is desired that the electrode or charge transferlayer have minimal electronic activity and instead serve primarily as alow resistance means for delivering the photogenerated current to theexternal circuitry or to the adjacent subsection of a multisectiondevice. Moreover, in PV devices, high contact or charge transfer layerresistance is detrimental in many applications since the resultingincreased series resistance limits power output.

[0047] The preferred embodiments of the present invention include, asone or more of the transparent electrodes of the optoelectronic device,a highly transparent, non-metallic, low resistance cathode such asdisclosed in Ser. No. 09/054,707, or a highly efficient, low resistancemetallic/non-metallic composite cathode such as disclosed in Forrest‘436. Each type of cathode is preferably prepared in a fabricationprocess that includes the step of sputter depositing an ITO layer ontoeither an organic material, such as copper phthalocyanine (CuPc), PTCDAand PTCBI, to form a highly transparent, non-metallic, low resistancecathode or onto a thin Mg:Ag layer to form a highly efficient, lowresistance metallic/non-metallic composite cathode. Ser. No. 09/054,707discloses that an ITO layer onto which an organic layer had beendeposited, instead of an organic layer onto which the ITO layer had beendeposited, does not function as an efficient cathode.

[0048] In summary, it is an object of the present invention to providean organic photosensitive optoelectronic device with two transparentelectrodes.

[0049] More specifically, it is an object of the present invention toprovide a stacked solar cell comprised of one or more subcells comprisedof two transparent electrodes.

[0050] It is another object of the present invention to provide astacked solar cell capable of operating with a high external quantumefficiency.

[0051] It is a still more specific object of the present invention toprovide a stacked solar cell capable of operating with an externalquantum efficiency that approaches the maximum internal quantumefficiency of an optimal PV subcell.

[0052] It is yet another object of the present invention to provide astacked solar cell capable of operating with a higher voltage than canbe provided by a single subcell.

[0053] Another object of the present invention is to provide an organicphotosensitive optoelectronic device including multiple quantum wellstructures.

[0054] A further object of the present invention is to provide a stackedorganic photosensitive optoelectronic device comprised of multipleorganic photosensitive optoelectronic subcells with the subcells havingexternal electrical connections.

[0055] Another object of the present invention is to provide an organicphotosensitive optoelectronic device with improved absorption ofincident radiation for more efficient photogeneration of chargecarriers.

[0056] It is a further objective of the present invention to provide anorganic photosensitive optoelectronic device with an improved V_(OC) andan improved I_(SC).

[0057] Another object of the present invention is to provide a stackedorganic photosensitive optoelectronic device having parallel electricalinterconnection of the subcells.

[0058] A further object of the present invention is to provide a stackedorganic photosensitive optoelectronic device comprised of multipleorganic photovoltaic subcells with transparent electrodes and having asubstantially reflective bottom layer to increase overallelectromagnetic radiation absorption by capturing the electromagneticradiation reflected by the bottom layer.

[0059] Yet another object of the present invention is to provide organicphotosensitive optoelectronic devices including a conductive or aninsulating substrate.

[0060] A further object of the present invention is to provide organicphotosensitive optoelectronic devices including a rigid or a flexiblesubstrate.

[0061] A further object of the present invention is to provide organicphotosensitive optoelectronic wherein the organic materials used arepolymeric or non-polymeric thin films.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The foregoing and other features of the present invention will bemore readily apparent from the following detailed description ofexemplary embodiments taken in conjunction with the attached drawings.It will be noted that for convenience all illustrations of devices showthe height dimension exaggerated in relation to the width.

[0063]FIG. 1 illustrates a simple prior art model of photoconductivityin organic materials.

[0064]FIGS. 2A, 2B, 2C and 2D schematically depict prior art organicphotovoltaic devices.

[0065]FIG. 3 schematically depicts an organic photosensitiveoptoelectronic device with transparent non-metallic electrodes and asingle organic layer.

[0066]FIGS. 4A and 4B schematically depict organic photosensitiveoptoelectronic device with transparent electrodes and two organiclayers.

[0067]FIG. 5 illustrates the energy levels of an isotype bilayerinterface.

[0068]FIG. 6 schematically depicts an organic photosensitiveoptoelectronic device with transparent non-metallic electrodes and morethan two organic layers.

[0069]FIG. 7 schematically depicts a stacked organic photosensitiveoptoelectronic device arrangement where each subunit has one organiclayer.

[0070]FIGS. 8A, 8B, 8C and 8D schematically depict stacked organicphotosensitive optoelectronic device arrangements where each subunit hastwo organic layers.

[0071]FIG. 9 schematically depicts a stacked organic photosensitiveoptoelectronic device arrangement where each unit has more than twoorganic layers.

[0072]FIGS. 10A and 10B schematically depict organic photosensitiveoptoelectronic devices employing multiple stacked photoconductivelayers.

[0073]FIGS. 11A and 11B schematically depict organic photosensitivedevices utilizing reflective layers.

[0074]FIG. 12 is a stacked photosensitive optoelectronic device withparallel interconnection of subunits after fabrication in a morerealistic depiction.

DETAILED DESCRIPTION

[0075] In an exemplary embodiment of the present invention, as shown inFIG. 3, the device structure 300 is deposited onto an insulatingsubstrate 301. First electrode 302 is transparent and comprises, e.g.,ITO deposited by conventional techniques or available predeposited oncommercial substrates to an approximate thickness of 1000-4000 Å,preferably less than 2000 Å and most preferably around 1000 Å. Layer 303is a single organic photoconductor, e.g., CuPc or PTCDA, or PTCBIdeposited by organic molecular beam deposition (OMBD) in a layer ofthickness 300-1000 Å, preferably about 500 Å. Second electrode 304 istransparent, e.g., ITO deposited by sputtering to an approximatethickness of 1000-4000 Å, preferably less than 2000 Å and mostpreferably around 1000 Å. An understanding from the prior art wouldindicate that such a symmetric unilayer device configuration would notgenerate a net photocurrent. However, the necessary asymmetry isbelieved to arise at the interface between organic photoconductor layer303 and second electrode 304 from electronic surface states in organicphotoconductor layer 303 produced during the deposition of ITO secondelectrode 304. The damage region is schematically represented as region303 a. The exact mechanism by which the altered region at the interfaceproduces asymmetric charge separating regions is not completelyunderstood. Without limiting the present invention to a particulartheory, the effect of energetic electrode deposition, such assputtering, may be sufficient to alter the energy states at theinterface so significantly that the conventional picture of opposingmirror-image Schottky barriers is altered. Forrest Appl. ‘707 disclosesthat the surface states, which may be defects, may effectively providesmall energy “steps” that allow electrons to more easily traverse whatwould otherwise be, for example, a Schottky barrier. One theory ofunilayer device operation is that deposition of electrode 304 ontoorganic layer 303 creates a low resistance contact, here a cathode. Itis believed in this case that the resulting asymmetry would yield a netphotocurrent.

[0076] Exemplary embodiments of an organic photosensitive optoelectronicdevice with two organic layers, or a bilayer device 400, are depicted inFIGS. 4A and 4B. The substrate 401 provides support for first electrode402 which comprises ITO of approximate thickness 1000-4000 Å, preferablyless than 2000 Å and most preferably around 1000 Å. First organic layer403 is adjacent to first electrode 402 and comprises, e.g., PTCDA orPTCBI. Second organic layer 404 comprising, e.g., CuPc, is adjacent tofirst organic layer 403 to form the photovoltaic heterojunction.Finally, in 4A00 in FIG. 4A, second electrode 4A05 comprises ITO ofapproximate thickness of 1000-4000 Å, preferably less than 2000 Å andmost preferably around 1000 Å and is deposited onto second organic layer404. In the alternate embodiment 4B00 in FIG. 4B, the second electrodeis a metallic/non-metallic composite electrode comprised ofsemitransparent metallic layer 4B05 which is topped by an ITO layer4B06. Semitransparent metallic layer 4B05 may preferably consist of 10%Ag in 90% Mg, or other low work function metals or metal alloys. Layer4B05 is thin, preferably less than 100 Å thick. ITO layer 4B06 is about1000-4000 Å thick and is preferably less than 2000 Å and most preferablyapproximately 1000 Å thick. Both organic layers are approximately 300-1000 Å thick and are preferably about 500 Å thick. The junction formedat the first organic layer 403/ second organic layer 404 interface issuch that under electromagnetic radiation, excitons formed in thevicinity of the interface are dissociated to form free electron-holepairs. Photogenerated holes are collected at first electrode 402 andphotogenerated electrons are collected at second electrode 405.

[0077] Embodiment 4A00 incorporates the low resistance metal substitutecathode disclosed in Forrest Appl. ‘707. It should be observed that incontrast to the OLEDs of Forrest Appl. ‘707 wherein the electrons movefrom the cathode into the adjacent organic semiconductor, organicphotosensitive optoelectronic bilayer device 400 represents anembodiment of a photosensitive optoelectronic device. Accordingly, thethicknesses of the photoconductive organic materials are adjusted to therequirements of a photosensitive optoelectronic device, and duringoperation electrons flow from second organic layer 404 into secondelectrode 4A05. Without being limited to this particular theory, it wastypically believed by one skilled in the art that metal substituteelectrodes, e.g., ITO, would create a barrier to electron flow in bothdirections, i.e., into or out of the device, when such materials wereused as cathodes. The low resistance operation of the ITO cathode in thepresent invention demonstrates that the surface states proposed inForrest Appl. ‘707 are effective in reducing the barrier to electronflow in either direction at an ITO cathode made in accord with thepresent invention of Forrest Appl. ‘707. Although first organic layer403 and second organic layer 404 may be interchanged in some cases andstill obtain a working device with only a reversal of polarity, theseembodiments are less preferred due to the reduced efficiency of a devicewhenever the metal substitute electrode is used as the bottom cathode.The surface states believed to effectively reduce the barrier at an ITOcathode are not thought to be produced when the organic thin film isdeposited onto the ITO layer. The term “low resistance metal substitutecathode” is, thus, used herein to refer to a metal substitute, e.g.,ITO, prepared by sputter depositing a metal substitute, e.g., ITO, ontoan organic layer, in particular, as distinct from an electrode preparedby depositing an organic layer onto a metal substitute, e.g., ITO. It isbelieved herein that the barrier-reducing effect of surface states alsocan function to permit holes to traverse what would otherwise beblocking junctions at an interface between an organic semiconductorlayer and an electrode or charge transfer layer.

[0078] Embodiment 4B00 utilizes a transparent metallic/non-metalliccomposite cathode comprised of layers 4B05 and 4B06. Here again, as anembodiment particularly suited for use as a solar cell, during operationelectrons generated near the 403/404 interface move toward layer 4B05and move through layer 4B06 for extraction. It is noted, in particular,that, whenever the ITO is sputter deposited onto a given thickness ofMg:Ag, the metallic/non-metallic composite cathode of Mg:Ag/ITO hassubstantially higher transparency than a noncomposite metallic layerhaving the same given thickness as the metallic part of the compositeelectrode. Further, the composite transparent composite layer may havebetter electronic properties than the lower resistance highlytransparent non-metallic cathode. Thus, while the transmission of themetallic/non-metallic composite electrode is typically not as high thehighly-transmissive low resistance metal substitute cathode, thetransmission is still sufficient, in combination with the superiorelectronic properties, for such metallic/non-metallic cathodes to beuseful in the photosensitive optoelectronic devices of the presentinvention.

[0079] In the bilayer cell, charge separation occurs in the vicinity ofthe organic heterojunction whose built-in potential is determined by thesize of the HOMO—LUMO gap energy difference between the contactingmaterials, here first organic layer 403 and second organic layer 404.FIG. 5 depicts the relative HOMO—LUMO gaps of exemplary materials for anisotype heterojunction, e.g., CuPc/PTCDA. The proximity of the Fermienergy, E_(F), to the HOMO band shows that all of these materials haveholes as the majority carrier. The HOMO offset is equal to thedifference of ionization potentials (IP) of the two contacting materialsand the LUMO offset is equal to the HOMO offset plus the difference inHOMO—LUMO gap energies (E_(H-L)) of the two materials. The polarity ofthe device is generally determined by the HOMO—LUMO energy differences.For example, in FIG. 5 since the LUMO of the PTCDA first organic layer403 is higher in energy than that in the CuPc second organic layer 404,electrons that are liberated from an exciton will move from the PTCDA403 into the CuPc 404 and contribute to the photocurrent. Absorption canoccur in either of the two organic films, thereby extending the width ofthe photoactive region where optical absorption can result in efficientcharge separation by the built-in field. Note that it is not criticalwhether a “p—n”-like junction or a simple isotype (i.e., p-P or n-N)heterojunction is employed, since it is only the diffusion potentialcreated at the heterojunction due to the HOMO—LUMO gap offsets that iseffective in carrier drift.

[0080] In the bilayer cells any rectifying effect of the organicmaterial/electrode junction is much less significant than that occurringat the bilayer heterojunction. Further, since the damage layer whichoccurs when, e.g., ITO, is deposited over an organic material issignificant in causing exciton ionization, this damage may be controlledusing the low power sputtering technique described in Forrest Appl. ‘863. It is believed that controlling this damage region will insure thatexciton ionization and carrier separation primarily occurs at theorganic/organic bilayer heterojunction.

[0081] An exemplary embodiment of a organic photosensitiveoptoelectronic cell with multiple organic layers, or a multilayer device600, is shown in FIG. 6. Insulating or conducting substrate 601 supportsthe device. First electrode 602 comprises, e.g., ITO of approximatethickness 1000-4000 Å, preferably less than 2000 Å and most preferablyaround 1000 Å and is adjacent to first organic layer 603 comprising,e.g., PTCDA, PTCBI, or CuPc of approximate thickness 20-50 Å. A secondorganic layer 604 comprises, e.g.4,4-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl (α-NPD), approximately200-500 Å in thickness, and is adjacent to first organic layer 603. Athird organic layer 605, comprising, e.g., aluminumtris(8-hydroxyquinoline) (Alq₃), approximately 200-500 Å in thickness,is adjacent to second organic layer 604 to form a rectifyingheterojunction at the second organic layer 604/third organic layer 605interface. A fourth organic layer 606, comprising, e.g., CuPc, PTCBI, orPTCDA, of approximate thickness 20-50 Å is adjacent to third organiclayer 605. Finally, second transparent electrode 607 is adjacent to thethird organic layer 605 and comprises, e.g., ITO of approximatethickness 1000-4000 Å, preferably less than 2000 Å and most preferablyaround 1000 Å. In this embodiment, an extra pair of organic materials,here second organic layer 604 and third organic layer 605, selected tohave appropriate relative mobilities and HOMO—LUMO offset for excitonionization and charge separation is placed within a “sandwich” of twoother organic materials, here first organic layer 602 and fourth organiclayer 606. In this instance, the “inner” pair of organic materials, 604and 605, provides the exciton ionization and charge separation and the“outer” pair, 603 and 606, serves both as charge transporting layers,i.e., transporting the separated carriers to the proper electrodes forsubstantially ohmic extraction, and as protective cap layers, i.e.,protecting the inner pair of organic layers from damage duringdeposition and use. The outer pair of organic materials may be from thegroup consisting of CuPc, PTCDA, and PTCBI, or any two of the three maybe used. That is, the same material or any combination thereof may beused for both contacts. Note, however, in embodiment 600, the interiorpair of layers, 604 and 605, are preferably deposited so that thecathode side is on top so as to incorporate a low resistance cathode.However, as with the exemplary embodiment of FIG. 4A, the order of thedeposition of the inner pair of organic materials is not criticalelectronically, though the order of the inner pair determines thepolarity of the photosensitive optoelectronic device . Since the outerpair of organic layers is relatively thin, their electronic propertiesare of much less significance here than in the bilayer exemplaryembodiment described herein above wherein the CuPc, PTCDA, and PTCBIalso performed photoconversion and exciton ionization in addition totransporting the separated carriers. Accordingly, an alternateembodiment of the present invention (not depicted) in a multilayerdevice would include the cathode on the bottom. The inner pair oforganic materials may each be an organic dye chosen to havephotosensitivity in a desired region of the spectrum. Since theAlq₃/α-NPD pair is photosensitive in the ultraviolet (UV) part of thespectrum, multilayer device 600 with this organic pair combination is aparticular exemplary embodiment of a UV photodetector. Further, the dyepair is preferably chosen to have a LUMO—HOMO gap offset as describedabove. In yet another embodiment (not shown) one or both of the outerpair of organic layers is replaced with a thin layer, approximately50-150 Å of Mg:Ag alloy which acts as a charge transfer, extraction, andprotective cap layer.

[0082] A further exemplary embodiment of the present invention is shownin FIG. 7, which depicts a stacked single-organic-layer (unilayer)photosensitive optoelectronic arrangement 700 wherein the individualunilayer subcells are electrically connected in parallel. Insulating orconductive substrate 701 provides support for the stacked photosensitiveoptoelectronic device. The stack shown consists of three completeunilayer photosensitive optoelectronic subcells separated by transparentinsulating layers. Specifically, each subcell has a first transparentelectrode, e.g., ITO, 702 a, 702 b, and 702 c. On top of each first ITOelectrode there are organic layers 703 a, 703 b, and 703 c. Then on topof each organic layer is a second ITO electrode 704 a, 704 b, 704 c.Since the damage layer which produces the required device asymmetry isalways developed at the interface where ITO is deposited onto an organicmaterial, all of the devices will have the same electrical polarity withrespect to the direction of material deposition. That is, the subcellsare inherently deposited in an electrically series configuration. Such aseries configuration may be useful in low power applications asdiscussed above. Also, in accord with the present invention, externalconnections are possible to the intervening electrodes to allow for theavailability of multiple voltages from a single device comprisingstacked unilayer subcells. (This embodiment is not illustrated).However, a parallel electrical arrangement is preferable for high powerapplications in view of the series resistance and space charge effectsdiscussed above. This means that the subcells may not share common ITOelectrodes as is possible in the bilayer stacks discussed below.Therefore, in the embodiment shown in FIG. 7, transparent insulatinglayers 705 a and 705 b are provided to separate the adjacent subcells.This allows all the first ITO electrodes, 702 a, 702 b, and 702 c to beconnected to a common electrical path. Similarly, all the second ITOelectrodes 704 a, 704 b, and 704 c are connected to a common electricalpath. It can be appreciated that the stack could be continued to includea number of subcells. Stacked unilayer devices may be optimizedaccording to the criteria disclosed herein to, e.g., maximize totalvoltage, quantum efficiency or power. Stacking beyond a thickness inwhich substantially all incident light is absorbed will not furtherincrease conversion efficiency.

[0083] Two exemplary embodiments 8A00 and 8B00 are shown in FIGS. 8A and8B. Embodiment 8A00 comprises several bilayer photosensitiveoptoelectronic devices arranged so that their polarity alternates as afimction of position in the stack. Insulating or conductive substrate801 provides support. Layers 802 a, 802 b, and 802 c are photosensitiveoptoelectronic subcell first transparent electrodes comprising, e.g.,ITO. They may also be some other transparent oxide or a metal. Layers803 a, 803 b, 803 c, and 803 d are photosensitive optoelectronic subcellfirst organic layers comprising, e.g., CuPc. Layers 804 a, 804 b, 804 c,and 804 d are photosensitive optoelectronic subcell second organiclayers comprising, e.g., PTCDA or PTCBI. Layers 805 a and 805 b arephotosensitive optoelectronic subcell second transparent electrodescomprising, e.g., ITO. Layer thickness ranges are the same as thosegiven for the individual bilayer subcell embodiment described withrespect to FIG. 4A. Since the subcells are deposited back-to-back, allelectrodes within the interior of the stack, i.e., 802 b and 805 a and805 b, are contacts to two different subcells and the entire stackarrangement is electrically connected in parallel. As used herein, theterm “back-to-back” is used to indicate that adjacent subcells haveantiparallel polarities. It can be appreciated here as well that thestack could be continued to include an arbitrary number of subcells toincrease the photocurrent. However, stacking beyond a thickness in whichsubstantially all incident light is absorbed will not further increaseconversion efficiency. Also, the choice of which organic layer todeposit first is not critical though it does determine the stackordering thereafter for a given stacked device.

[0084] In another exemplary embodiment 8B00 of a stacked device ofseveral back-to-back subcells, electrode layers 802 a, 802 b and 802 care augmented as now described. In order to take advantage wherepossible of the beneficial effect of a thin metallic layer combined witha layer of ITO, using what is herein referred to as ametallic/non-metallic composite electrode, thin semitransparent metalliclayers, e.g., Mg:Ag, are added in several places. Metallic layer 8B06 isadjacent to ITO layer 802 a. Metallic layers 8B07 and 8B08 are adjacentto ITO layer 802 b. Metallic layer 8B09 is adjacent to ITO layer 802 c.

[0085] It is appreciated that in the electronically back-to-backconfiguration of stacked bilayer device 8A00, the subcell cathodesnecessarily are alternately on top of the individual subcells as thestack is built from bottom up. This means that not all subcells willhave what is referred to herein as the low resistance non-metalliccathode. In yet another embodiment (not shown), the subcells may bedeposited with the organic layers in non-alternating order as justdescribed but with additional transparent electrode layers andinsulating layers between the individual subcells in a manner similar tothat described above for the unilayer device. In this embodiment, thesubcells may be connected externally in parallel and also have thecathode always on top to take advantage of the low resistancenon-metallic cathode.

[0086] In other embodiments, a bilayer stacked device may be constructedin which the subcells are in series by not alternating the order of thetwo organic layers. In accord with the present invention such devicesmay have external connections to electrodes between subunits or may havethe subunits interconnected by charge transfer layers and isolated fromexternal circuits. FIG. 8C depicts a series connected device 8C00 withintervening electrodes. Layers 803 a, 803 b, 803 c, 803 d are arrangedto be on top of layers 804 a, 804 b, 804 c and 804 d in each of theirrespective subcells. This means that each subcell is arranged to have anETL, e.g., CuPc, on top of a HTL, e.g., PTCDA or PTCBI, so that eachsubcell has the cathode on top. This means that this embodiment isarranged to take advantage of the low resistance non-metallic cathode.Device 8C00 provides a plurality of negative voltages V_(L1 . . . N)where N is the number of subcells. N=4 is depicted in FIG. 8C. Note alsothat each available voltage V_(Li) is the sum of the voltages from alllower numbered cells. In another embodiment (not shown) which is avariant of device 8C00, the metallic/non-metallic composite cathode isutilized by inserting thin semitransparent metallic layers of, e.g., 10%Ag in 90% Mg, or other low work function metals or metal alloys. Thesemetallic layers of, approximately 100 Å or less in thickness, are placedbelow and adjacent to the metal substitute, e.g., ITO, layers 805 a, 802b, 805 b and 802 c to form a metallic/non-metallic composite cathode foreach subunit. In another embodiment (not shown) layers 804 a, 804 b, 804c and 804 d are arranged to be on top of layers 803 a, 803 b, 803 c, 803d so that the subunits have the anode on top and the voltages V_(Li)correspondingly have the opposite polarity. FIG. 8D depicts a seriesconnected device 8D00 similar to device 8C00 but with the subunitsinterconnected by charge transfer layers 8D10, 8D11 and 8D12 of, e.g,ITO of approximate thickness 1000-4000 Å, preferably less than 2000 Åand most preferably around 1000 Å. There is yet another embodiment (notshown) in which the organic semiconducting layers are reversed so that adevice with an overall opposite electrical polarity is obtained. Anotherembodiment is a variation (not shown) of device 8D00 in which thinsemitransparent metallic layers of, e.g., 10% Ag in 90% Mg, or other lowwork function metals or metal alloys layers of approximate thickness 100Å Å or less are placed below and adjacent to layers 8D10 , 8D11 and 8D12to form metallic/non-metallic composite charge transfer layers and belowand adjacent to layer 802 c to form a metallic/non-metallic compositecathode.

[0087] In FIG. 9, an exemplary embodiment 900 of a stacked organicphotosensitive optoelectronic device comprised of a number of multilayerorganic photovoltaic subcells. Insulating or conductive substrate 901provides support. Photovoltaic subcell first transparent electrodes 902a, 902 b, and 902 c comprise, e.g., ITO. Photosensitive optoelectronicsubcell first organic layers 903 a, 903 b, 903 c, and 903 d comprise,e.g., CuPc or PTCDA or PTCBI. Photosensitive optoelectronic subcellsecond organic layers 904 a, 904 b, 904 c, and 904 d comprise, e.g.,α-NPD. Photosensitive optoelectronic subcell third organic layers 905 a,905 b, 905 c, and 905 d comprise, e.g., Alq₃. Photosensitiveoptoelectronic subcell fourth organic layers 906 a, 906 b, 906 c, and906 d comprise, e.g., CuPc or PTCDA or PTCBI. Photosensitiveoptoelectronic subcell second transparent electrodes 907 a and 907 bcomprise, e.g., ITO. Dimensions in each subcell of this exemplaryembodiment are the same ranges as in the embodiment of FIG. 6 but arevaried for optimization according to the criteria disclosed elsewhereherein. All first transparent electrodes 902 a, 902 b, and 902 c areelectrically connected in parallel and all second transparent electrodesare electrically connected in parallel to provide increasedphotocurrent. Here as well, the stack could be continued to include anarbitrary number of subcells. However, stacking beyond a thickness inwhich substantially all incident light is absorbed will not furtherincrease conversion efficiency. Just as the order of the inner pairs ofthe subcell was not critical in the exemplary embodiment of FIG. 6, itis not critical here either except that the pair order must alternatebetween subcells to allow the interior transparent electrodes to act ascommon contacts for adjacent cells. Therefore, the order of the firstinner pair of organic layers will determine the order for the remainderof the stack in this parallel interconnected embodiment.

[0088] A further embodiment of the present invention (not shown) is avariation of embodiment 900 and has the subcells separated bytransparent insulating layers, such as SiO₂ of approximate thickness500-4000 Å, preferably less than 3000 Å and most preferably around 2000Å, with additional transparent electrode layers as required in analogyto the unilayer and bilayer stacked devices described above. If the Alq₃and α-NPD layers are arranged with the Alq₃ on top then the subcells areall oriented with the cathode on top to utilize the low resistancecathode such as disclosed in Forrest co-pending Appl. ‘707. With thesubcells thus electrically isolated in the stack they may be connectedexternally in accord with the present invention in either a parallel orseries manner as described with the bilayer subcells.

[0089] In other variations of embodiment 900, the present invention hasa stacked configuration wherein the subunits are stacked in series,i.e., with aligned polarity but not separated by insulating layers. Inone embodiment (not shown) the multilayer cells are interconnected byelectrode layers such that a plurality of voltages are available from asingle device similar to embodiment 8C00. In another variation (notshown) the adjacent multilayer subcells are interconnected by chargetransfer layers in a manner similar to embodiment 8D00. In all of thevariations (not shown) of embodiment 900 described herein, themetallic/non-metallic composite cathode such as disclosed in Forrest‘436 may be used in place of the highly transparent, low resistancenon-metallic cathode. In this case the fourth organic layer of eachsubcell, 906 a, 906 b, 906 c, and/or 906 d, may be eliminated, ifdesired.

[0090] In yet another embodiment (not shown) one or both of the outerpair of organic layers in the subcells is replaced with a thin layer,approximately 50-150 Å, of Mg:Ag alloy which acts as a charge transfer,extraction, and protective cap layer.

[0091] It should be noted that in all of the stacked devices so fardiscussed, e.g., unilayer, bilayer, and multilayer, the thicknesses ofall the subunits in any given device may be uniform, i.e., substantiallythe same from one subcell or subunit to the next. In particular, thethicknesses of the photoconductive organic layers which performphotoconversion are approximately the same in each subunit. However,since the photosensitive layers absorb electromagnetic radiation, theintensity of radiation admitted on one side, e.g., the top, of a stackeddevice decreases as the radiation traverses deeper into the device. Thevoltage generated by each cell or subcell is generally a function ofcertain intrinsic properties, e.g., the energy level distributions inthe respective materials forming the heterojunction and in particularthe HOMO—LUMO gap as previously discussed herein. However, the currentgenerated in a cell or subcell depends on the amount of electromagneticradiation which is admitted to the particular cell or subcell. In astacked device composed of multiple subcells of uniform thickness andexposed to ambient electromagnetic radiation from only one direction,the current generated by subcells farther from the admission surface, orface, of the stacked device will decrease in cells progressively fartherfrom the face. In stacked devices which are configured to beelectrically connected in parallel, e.g., embodiments 8A00 and 900, thiswould not necessarily be a major drawback since the currents arecombined externally to supply a current equal to the sum of that fromall subcells with the voltage fixed by the intrinsic uniform voltage ofthe subcells. In such parallel devices the total power output of eachsubcell may be extracted without any inefficiency due to limitation by aparticular subcell. In situations where it is desirable to obtainmultiple outputs having the same current capacity, devices may beconstructed as described below.

[0092] On the other hand, the different current limitation in eachsubcell may be an important factor in the operation of series connectedstacked devices. In a stacked device which is connected in series suchas embodiment 8D00, fundamental current continuity considerationsconstrain the device's current output so that it is limited to thecurrent which goes through the subcell generating the least currentregardless of the relative position of a subcell in the stack. Toaddress this problem, other embodiments of the present invention utilizesubcells having photoconductive organic layers with thicknesses whichvary systematically between subcells. For example, in a variation ofembodiment 8D00, the photoconductive organic layers vary in thicknesssuch that each subcell has exponentially thicker photoconductive organiclayers if measured starting at the top of the device. This lastvariation of embodiment 8D00 would be most appropriate where the sourceof the electromagnetic radiation is limited to one side of the device,here the top, i.e., farthest from the substrate. In another variation,the photoconductive organic layers may be made exponentially thickerstarting from the bottom subcell. This embodiment would be appropriatein an environment where the source of electromagnetic radiation is onthe substrate side of the device when the substrate is transparent. Foruniformly illuminated environments where approximately equal intensityelectromagnetic radiation light is incident on the two faces of adevice, yet another variation of embodiment 8D00 may be efficient. Ifthe electromagnetic radiation has sufficient intensity that asignificant amount of the radiation incident upon each face is able totraverse the device, then an embodiment variation of 8D00 wherein thephotoconductive organic layers in the subcells in the center are thickerthan corresponding layers in subcells nearer the two faces will provideuniform current levels from each cell.

[0093] The situation with series stacked devices which generate multipleindependently accessible voltages, e.g., 8C00, is somewhat morecomplicated. In such a device, the currents through different subcellsin the device may be different since the multiple external connectionsto the various internal electrodes allow multiple paths for current toflow so the effect of low current production by any particular subcelldepends on the load dynamics.

[0094] In other embodiments of the present invention multiple layers oforganic photosensitive materials are used to enhance device sensitivity.In one embodiment, organic photosensitive optoelectronic devices inaccord with the present invention incorporate multilayer photoconductorlayers to enhance exciton dissociation. Specifically it is believed thatthe presence of numerous well defined interfaces between materialsprovide more extensive exciton dissociation regions. The compositelayers may have thicknesses commensurate with the device guidelinesdescribed above or thinner layers as described next.

[0095] In yet a further embodiment, organic photosensitiveoptoelectronic devices in accord with the present invention incorporatemultiple quantum wells (MQWs). MQWs are believed to alter thedistribution of exciton energy levels resulting in, inter alia, possiblechanges in the spectral sensitivity of MQW-containing devices ascompared to devices having the same materials and thicker photosensitivelayers that do not exhibit quantum size effects. Also, it is believedthat the changes in exciton energy levels may serve to enhance excitondissociation. It is believed that these embodiments having multiplephotosensitive layers including MQW structures provide enhancedphotosensitivity but may not be appropriate for continuous use sincethey can quickly saturate due to charge which becomes trapped in thelayers. Therefore, in an exemplary use organic photosensitiveoptoelectronic devices in accord with the present invention andincorporating MQWs would be used as intermittant photodetectors with,for example, a liquid crystal shutter mechanism which would periodicallyblock light for short intervals from the detector so as to allow thetrapped charge to dissipate. Such structures may be operated with apurely passive load as depicted in FIGS. 10A and 10B. However, thesemultilayer structures are highly efficient when operated with an appliedbias. Under such conditions such a device can produce a large transientcurrent when initially exposed to light.

[0096] Arbour et al. describes bilayer and multilayer assemblies ofvanadyl phathalocyanine (VOPc) and PTCDA using semitransparent Au orSnO₂ thin films as one contact, and transparent electrolytes, saturatedin a redox species for the opposing electrical contact. It isappreciated that Arbour did not utilize solid state electrode materialssuitable for practical devices for both electrical contacts. Also, it isappreciated that Arbour's multilayer devices did not have interveningelectrodes or charge transfer layers as disclosed herein above. ForrestChem. Rev further describes the effect of multiple layers of organicphotosensitive materials but does not disclose the use of such layers indevices having two transparent electrodes.

[0097] Embodiments with numerous well defined interfaces and/or MQWs arenow described with reference to FIG. 10A and FIG. 10B. FIG. 10Aschematically depicts a device 10A00 with two electrodes 10A02 and10A05, which are, e.g., ITO of approximate thickness 1000-4000 Å,preferably less than 2000 Å and most preferably around 1000 Å. For thisembodiment, the bottom ITO electrode preferably functions as the anodeand the top electrode functions as the cathode. For example, the top ITOelectrode may be sputter deposited onto the underlying organic layer soas to form a low resistance non-metallic cathode or, alternatively, thetop electrode 10A05 may be comprised of a thin Mg:Ag layer onto whichthe ITO layer is sputter deposited so as to form a metallic/non-metalliccomposite cathode. The layers 10A03 (a, b, c, d, e, f, g, h, i, and j)and 10A04 (a,b, c, d, e, f, g, h, i, and j) are pairs of organicphotoconductive materials which form a plurality of interfaces betweeneach respective 10A03 and 10A04 pair. The layers may be organicmolecular crystal semiconducting materials as described elsewhere hereinor polymer semiconductors as also described herein. For example, in oneembodiment the pairs may be, e.g., PTCDA/CuPc, PTCBI/CuPc, orPTCDA/VOPc. Note also that the layer pairs need not necessarily be ofdifferent majority carrier type but may be isotype. The importantfeature of the layer pairs is that they produce numerous interfaces topromote and in some cases alter exciton dissociation dynamics. Layers ofthe exemplary materials just mentioned may be in the range ofapproximately 20-1000 Å with 500 Å being the preferred upper end of therange. Devices designed specifically to utilize MQW effects will havemuch thinner layers with thicknesses determined by the approximatespatial dimensions of the primary excitonic modes. These considerationsare discussed in Forrest Chem. Rev. 1997 which is incorporated herein byreference. For the materials mentioned above, PTCDA/CuPc, PTCBI/CuPc,and PTCDA/VOPc, MQW layers should be in the 20-50 Å range. Embodiment10B00 in FIG. 10B is a variation of 10A00 having an additional thinmetallic layer 10B00 of, e.g., 50-150Å of Mg:Ag alloy which acts as acharge transfer, extraction, and protective cap layer and forms ametallic/non-metallic composite electrode with the metal substitute ITOlayer 10A05.

[0098] It is appreciated that in these devices the photoconductive layermaterials may be deposited with either member of the pair on top as longas the layers are alternated throughout the device. Further, it isappreciated that the number of pairs depicted is illustrative only andmay be more or fewer depending upon the thicknesses of thephotoconductive layers. The general constraint again is that the totalthickness of all the layers should not generally be greater than thepenetration length of the electromagnetic radiation for which the deviceis to be utilized.

[0099] In yet a further exemplary embodiment, a reflective substrate of,e.g., metal, or a non-reflective substrate coated with a reflectivelayer of, e.g., metal such as Al or Au, may be used to support organicphotosensitive optoelectronic device structures in accordance with thepresent invention. FIG. 11A depicts an example of an embodiment with areflective surface based on a single bilayer cell. 11A01 is a substrate,which may be transparent, e.g., glass, or non-transparent, e.g., ametal. Thin metal layer 11A02 is deposited on the substrate. Layer 11A03is an optional metal substitute electrode layer, for example, ITO. Iflayer 11A03 is not used then layer 11A02 functions as an electrode andis of a suitable material as described elsewhere herein. First organiclayer 11A04 is an organic semiconductor, for example, CuPc. Secondorganic layer 11A05 is a second organic semiconductor, for example,PTCDA or PTCBI. The transparent electrode 11A06 that is placed on top ispreferably a low resistance non-metallic cathode or ametallic/non-metallic composite cathode. FIG. 11B shows a stackedbilayer device embodiment with a reflective bottom layer with thesubcells connected in parallel. 11B01 is a substrate, which may betransparent, e.g., glass, or non-transparent, e.g., a metal. If a glasssubstrate is used, thin metal layer 11B02 may be deposited on thesubstrate. Since a metal substrate or metal layer is present to providethe reflective surface, the metal substrate or metal layer alsopreferably functions as the bottom electrode of the device. However, ifdesired, an optional transparent electrode layer 11B03 a comprised ofITO may be present. Layers 11B03 (b andc) are subcell first electrodelayers and may be a metal substitute such as ITO, a transparent metal,or comprise ITO over a thin metallic layer, such as Mg-Ag. Layers 11B06(a and b) are subcell second electrode layers and may be a metalsubstitute such as ITO, a transparent metal, or comprise ITO over a thinmetallic layer, such as Mg-Ag. Organic layers 11B04 (a, b, c and d) area first organic semiconductor, e.g., CuPc. Organic layers 11B05 (a, b, cand d) are a second organic semiconductor, for example, PTCDA or PTCBI.In another embodiment (not depicted), a transparent substrate is used tosupport any organic photosensitive optoelectronic device in accord withthe present invention. In this embodiment the top subcell of the stackhas a top reflective layer, typically and all intervening subcells havetransparent electrodes and light is received through the substrate. Thetop reflective layer can be an electrode or a metallic layer depositedon a transparent non-metallic electrode. In all of these embodiments,i.e., with either a top or a bottom reflecting surface, the reflectivesurface redirects any unabsorbed light back through the absorbing layersto allow further opportunity for photoconversion and enhancement of thequantum efficiency. Inclusion of a reflective layer reduces the numberof subcells required in a stack to obtain optimal photoconversionefficiency.

[0100] In fabricating practical organic photosensitive optoelectronicdevices, in addition to determining the optimal number of and thicknessof layers to have in a stacked photosensitive optoelectronic device asdiscussed above, the area and arrangement of individual cells may bechosen to optimize efficiency and cost. Since the transparent electrodesthat are to be used in these devices do have some residual absorption,it is preferable to keep such electrodes thin to minimize thisabsorption. In addition, keeping the electrode layers as thin aspracticable minimizes fabrication time. On the other hand, as allelectrode materials are made thinner, their sheet resistance increases.Therefore, it is preferable to minimize the distance which chargecarriers must travel in the electrode after collection. A configurationwhich maximizes photosensitive optoelectronic receptive area andaccomplishes this goal is one in which the devices are laid out in longstrips on the substrate with electrical connection made to the deviceselectrodes along the longer sides. U.S. patent application Ser. No.08/876666 to Forrest et al. (hereinafter Forrest Appl. ‘876), which isincorporated herein by reference in its entirety, describes techniquesfor fabrication of practical organic thin film devices.

[0101] Forrest Appl. ‘707 disclosed that small molecular organicmaterials with relatively large planar molecules and a highly conjugatedstructure, such as CuPc and PTCDA, are believed to be able to withstandenergetic electrode deposition because of the highly delocalized natureof their bonding which spreads the energy of impact over more than onebond. When an energetic metal or oxygen atom is incident on one of thesemolecules at a surface during sputtering, the energy of impact isthought to be efficiently distributed over the numerous bonds in themolecular π-electron systems. In contrast, no comparably largeπ-electron systems exist in, e.g., Alq₃ or α-NPD. For such molecules,the impact energy is more localized among only a few atomic sites,thereby increasing the probability for breaking a molecular bond. Theplanar or nearly planar stacking arrangements of crystalline molecularsystems such as CuPc and PTCDA may also assist in the dissipation ofenergy among several neighboring molecules in the lattice. Accordingly,it is believed that the low resistance non-metallic cathode such asdisclosed in Forrest Appl. ‘707 can also be embodied in other similartypes of small molecular crystals. For example, CuPc may be replaced byother metal phthalocyanines, naphthalocyanines and porphyrins. PTCDA maybe replaced for example by other polyacenes. Some other representativephotosensitive optoelectronic small molecule organic heterostructurecombinations contemplated as suitable for use in accord with the presentinvention are shown in Table 1. TABLE 1 Ag/PTCBI/CuPc/ITOIn/PTCDA/CuPc/ITO Au/H₂Pc/DM-PTCDI/ITO Au/H₂Pc/DM-PTCDI/PTCBI/ITOAu/H₂Pc/PTCBI/ITO Al/ZnPc/Au Au/ZnPc/DM-PTCDI/ITO In/PPEI/PVP(TPD)/ITOAu/CuPc/DM-PTCDI/ITO Au/ZnPc/DM-PTCDI/ITO Au/H₂Pc/PTCBI/ITOAu/TPP/DM-PTCDI/ITO Au/TBP/DM-PTCDI/ITO Au/H₂Hc/DM-PTCDI/ITOAu/H₂Pc/DM-PTCDI/ITO (Au/H₂Pc/DM-PTCDI)₂/ITO Au/(H₂Pc/DM-PTCDI)₂/ITOAl/C₆₀/TiOPc/ITO Al/C₆₀/VOPc/ITO Al/C₆₀/PPV/ITO Al/merocyanine/Ag****************************************************

[0102] PPEO: 3,4,9,10-perylenetetracarboxyl-N,N-bis(phenylethylimide).

[0103] PVP(TPD): 55 wt % N,N′-diphenyl-N,N′-ditolylbenzidine inpoly(vinylpyridine) spin coated onto ITO surface prior to PPEIdeposition.

[0104] TPP: 5,10,15,20-21 H,31H-tetraphenylporphyrin.

[0105] TBP: tetrabenzoporphyrin(29H,31H-tetrabenzo[2,3-b:2′3′-g2″,3″-1:2″’,3″’‘-q]porphyrazine).

[0106] H₂Nc: naphthalocyanine(29H,31H-tetranaphtho[2,3-b:2′3′-g:2″,3″-1:2″’,3″’-q]porphyrazine).

[0107] H₂Pc: phthalocyanine

[0108] PPV: poly(phenylene vinylene).

[0109] ZnPc: zinc phthalocyanine

[0110] DM-PTCDI:

[0111] TiOPc: titanium oxide phthalocyanine

[0112] C₆₀: buckminsterfullerene

[0113] VOPc: vanadyl phthalocyanine

[0114] Additionally, organic polymers may be used in accord with thepresent invention. Yu, G., Gao, J., Yang, C., and Heeger, A.,“Photovoltaic Cells Made with Organic Composites”, Proceedings of the1st NREL Conference on Future Generation Photovoltaic Technologies,American Institute of Physics, March 1997, incorporated herein byreference in its entirety, discloses polymer-based solar cells usingpure poly(2-methoxy-5-(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene)(MEH-PPV) and MEH-PPV blended with buckminsterfullerene (C₆₀). Thesematerials are believed to be suitable as photoconducting materials inaccord with the present invention. Also, “The Handbook of ConductingPolymers” Skotheim, T, Ed., Marcel Dekker, 1986, especially Chapter 17:Polymeric Semiconductor Contacts and Photovoltaic Applications” Kanicki,J, discloses a number of photoconductive polymers which are known in theart and which are believed to be suitable for use in devices made inaccord with the present invention. These include poly-2-vinylpyridine(PVP), polyphenylacetylene (PPA), polyphenylene (PPP), polyphenylenesulfide (PPS), polypyrrole (PPY), polyacrylonitrile (PAN),polyheptadiyne (PHD), polymethylacetylene (PMA), polyphenylene vinylene(PPPV), polyphenylene oxide (PPPO). These materials may be incorporatedin any of the several stacked photocell embodiments disclosed above.Also, it is believed that polymers that have a highly connectedstructure can form the low resistance non-metallic cathode by receivingan energetically deposited metal substitute cathode, such as sputteredITO, when such an electrode/semiconductor interface would otherwise forma substantial barrier to carrier flow.

[0115] Also, it will be appreciated that the novel concept ofelectrically connecting organic photosensitive optoelectronic devices inparallel to overcome the inefficiencies associated with their highseries resistance and related space charge effects is equally applicableto other organic photosensitive optoelectronic devices comprising othersubcell compositions. Similarly the novel concept of connecting stackedorganic photosensitive optoelectronic devices in series with externalconnections to intervening electrodes to provide a multivoltage powersupply is applicable to other organic photosensitive optoelectronicdevices. Also, use of the low resistance non-metallic cathode disclosedin co-pending Forrest Appl. ‘707 in place of a traditionalsemi-transparent metallic cathode, such as disclosed in prior artorganic photosensitive optoelectronic material configurations, isintended to be within the scope of the present invention.

[0116] General techniques for fabrication of devices in accord with thepresent invention are well-known in the art. The preferred method ofdepositing the organic layers is by thermal evaporation; the preferredmethod of depositing metal layers is by thermal or electron-beamevaporation; the preferred method of depositing dielectrics is byplasma-enhanced chemical vaporization, RF or RF/DC sputtering, orelectron-beam evaporation. The fabrication of the organic photosensitiveoptoelectronic devices may be accomplished, for example, with either ashadow masking technique or shadow masking combined with a dry etchingprocess for preforming the substrate. Shadow masking and dry etching areknown in the art. Advanced photolithographic deposition techniques forproduction of the various embodiments of the present invention have beenpreviously disclosed in relation to the fabrication of organic thinfilms in Forrest Appl. ‘666. For example, one of ordinary skill in theart can construct the device embodiment as depicted in FIG. 12 using thetechniques described in Forrest Appl. ‘666 and incorporated herein withthe further benefit of this disclosure.

[0117]FIG. 12 depicts an implementation of a stacked photosensitiveoptoelectronic device having three subcells with subcells connectedelectronically in parallel. Embodiment 1200 is depicted within the“shadow box” after the step of encapsulation as described in ForrestAppl. ‘666. 1201 is a continuous region of a transparent conductivematerial, e.g., ITO which forms a bottom electrode and one electrode inthe middle of the stack. Layer 1202 is a plurality of photoconductiveorganic layers (depicted as one here for convenience) which form arectifying junction in accord with the present invention. 1203 isanother continuous region of transparent conductive material, e.g., ITOwhich forms a top electrode and one electrode in the middle of thestacked device. Layers 1204 and 1205 are similarly pluralities ofphotoconductive organic layers. It is appreciated that in thisembodiment there are three subcells which are deposited so as to havealternating polarity, i.e., the top and bottom subcells have the samepolarity and the middle subcell has the opposite polarity. It isappreciated in embodiment 1200 that the photoconductive organic layers1202, 1204 and 1205 are intended to be continuously connected. Theorganic materials contemplated for use in this particular embodiment aresufficiently poor conductors that they can effectively insulate thetransparent conductive regions 1201 and 1203 from each other as depictedand still perform the photoconductive functions described elsewhereherein.

[0118] Protective layer 1206 should be made of a nonconductive material.For example, the protective layer could be spin coated polymer, such asTEFLON™, or sputter deposited SiO₂ or Si₃N₄. Protective layer 1206should also be transparent when electromagnetic radiation is to beadmitted through the top of the device. In another optional embodiment(not illustrated), protective layer 1206 may be omitted but the topelectrode layers must be made thick as to be impervious to ambientatmosphere to protect the organic materials comprising the device fromperformance degrading chemical reactions.

[0119] Using the “shadow box” technique a stacked bilayer organicphotosensitive optoelectronic device similar to FIG. 12 can be made inthe following steps:

[0120] 1) Depositing a transparent 5-10 μm dielectric layer onto asubstrate with prepatterned contacts. The dielectric layer can be SiO₂,for example.

[0121] 2) Depositing a photoresist layer.

[0122] 3) Exposing the photoresist layer to light in a pattern fordevice bottom layers.

[0123] 4) Removing unexposed photoresist areas to leave a pattern ofphotoresist over the dielectric layer.

[0124] 5) Remove the dielectric layer by, e.g., chloride reactive-ionetching to leave strips of dielectric covered with photoresist and wetetching to create the “undercut”.

[0125] 6) Angle deposit a first ITO layer.

[0126] 7) Angle deposit a CuPc layer.

[0127] 8) Angle deposit a PTCDA layer.

[0128] 9) Angle deposit a second ITO layer.

[0129] 10) Angle deposit a PTCDA layer.

[0130] 11) Angle deposit a CuPc layer.

[0131] 12) Angle deposit a second ITO layer.

[0132] 13) Repeat steps 7-12 to build up stack. The stacking can bestopped on repetition of either step 9 or 12 .

[0133] Using a conventional shadow masking technique a stacked bilayerorganic photosensitive optoelectronic device similar to FIG. 12 can bemade in the following steps:

[0134] 1) Deposit a first ITO layer by shadow masking onto a substratewith prepatterned contacts.

[0135] 2) Deposit a CuPc layer by shadow masking.

[0136] 3) Deposit a PTCDA layer by shadow masking.

[0137] 4) Deposit a second ITO layer by shadow masking.

[0138] 5) Deposit a PTCDA layer by shadow masking.

[0139] 6) Deposit a CuPc layer by shadow masking.

[0140] 7) Deposit a second ITO layer by shadow masking.

[0141] 8) Repeat steps 2-7 to build up stack. The stacking can bestopped on repetition of either step 4 or 7.

[0142] Forrest Appl. ‘666 also describes techniques for fabricatingarrays of optoelectronic devices which are also applicable to thepresent invention. With these techniques described in Forrest Appl. ‘666and this disclosure one of ordinary skill can construct an array (notdepicted) of organic photosensitive optoelectronic devices which isparticularly suitable as a multipixel photodetector. Such an array iscapable of detecting electromagnetic radiation with spatial andintensity resolution.

[0143] It will also be appreciated that the substrate may be any smoothmaterial such as transparent materials glass, quartz, sapphire orplastic, or opaque materials such as silicon or metal and includingflexible material such as plastic films, e.g., polystyrene, or metalfoils, e.g., aluminum foil. Any roughness present on the surface of thesubstrate can be smoothed by depositing an optional thin layer ofpolymer on top of the substrate, or by applying a similar smoothingtechnique. Though the exemplary embodiments generally describe organicphotosensitive optoelectronic devices deposited onto insulatingsubstrates, it will be appreciated that if a conductive metal is used asthe substrate it may be used as a first electrode in lieu of an appliedfirst electrode. Alternatively, an insulating layer of, e.g., aninsulating oxide, may be placed over the metal before the device isdeposited as described in the above exemplary fabrication method.

[0144] The organic photosensitive devices of the present invention maybe incorporated into, inter alia: light powered radios, televisions,computers, calculators, telephones and other wireless communicationsdevices, watches, emergency location devices, electric vehicles, powergeneration systems and devices, and emergency power supplies; monitoringand detection equipment for power and/or sensing, inspection devices,radiation detectors, imaging devices; A and optical coupling devices forelectrical isolation or use in fiber optic communications.

[0145] Those skilled in the art will recognize that the devicesdescribed above may be used as solar cells or photodetectors. That is,when such a device is exposed to electromagnetic radiation while a biasvoltage is applied, the device is in photodetector mode and a current isproduced that correlates with the intensity of the incident radiation.In solar cell operation, no bias voltage is applied and power isdirected through an external load. Accordingly, the present inventionand claims are intended to cover these devices regardless of which ofthese modes is used when operating the device.

[0146] Thus, there has been described and illustrated herein an organicphotosensitive optoelectronic device and method for producing the same.Those skilled in the art, however, will recognize that manymodifications and variations besides those specifically mentioned may bemade in the apparatus and techniques described herein without departingsubstantially from the concept of the present invention. Accordingly, itshould be clearly understood that the form of the present invention asdescribed herein is exemplary only and is not intended as a limitationon the scope of the present invention.

What is claimed is:
 1. An organic photosensitive optoelectronic devicecomprising: a substrate having a first major surface and a second majorsurface; at least two transparent electrode layers in superposedrelationship upon said first major surface of said substrate; and atleast one photoconductive organic layer disposed between said at leasttwo transparent electrode layers.
 2. The device of claim 1 wherein thethickness of said at least one photoconductive organic layer is selectedto maximize the external quantum efficiency of said device.
 3. Thedevice of claim 1 wherein the thickness of said at least onephotoconductive organic layer is selected to maximize the total currentoutput of said device.
 4. The device of claim 1 wherein the thickness ofsaid at least one photoconductive organic layer is selected to maximizethe fill factor of said device.
 5. The device of claim 2 wherein atleast one of said transparent electrode layers comprises a non-metallicconductive layer.
 6. The device of claim 2 wherein at least one of saidtransparent electrode layers comprises a metallic conductive layer. 7.The device of claim 5 wherein said non-metallic conductive layer is aconductive oxide.
 8. The device of claim 7 wherein said conductive oxideis selected from the group consisting of indium tin oxide, tin oxide,gallium indium oxide, zinc oxide and zinc indium oxide.
 9. The device ofclaim 8 wherein said conductive oxide is indium tin oxide.
 10. Thedevice of claim 5 wherein said non-metallic conductive layer is aconductive polymer.
 11. The device of claim 10 wherein said conductivepolymer is polyanaline.
 12. The device of claim 5 wherein at least oneof said transparent electrode layers further comprises a metallic layerdisposed between said non-metallic conductive layer and said at leastone photoconductive organic layer.
 13. The device of claim 2 wherein atleast one of said transparent electrode layers is a low resistancenon-metallic cathode.
 14. The device of claim 13 wherein said lowresistance non-metallic cathode comprises indium tin oxide.
 15. Thedevice of claim 2 wherein at least one of said transparent electrodelayers is a metallic/non-metallic composite cathode.
 16. The device ofclaim 15 wherein said metallic/non-metallic cathode comprises indium tinoxide and magnesium silver.
 17. The device of claim 12 wherein saidmetallic layer comprises a metal selected from the group consisting ofgold, aluminum, magnesium, indium, and silver.
 18. The device of claim12 wherein said metallic layer is an alloy consisting essentially ofmagnesium and silver.
 19. The device of claim 1 wherein said at leastone photoconductive organic layer comprises an organic molecular crystalmaterial.
 20. The device of claim 19 wherein said organic molecularcrystal material is selected from the group consisting of phthalocyaninecompounds, perylene compounds, polyacene compounds, and porphyrincompounds.
 21. The device of claim 1 wherein said at least onephotoconductive organic layer comprises a polymeric material.
 22. Thedevice of claim 1 wherein said at least one photoconductive organiclayer is one photoconductive organic layer.
 23. The device of claim 1wherein said at least one photoconductive organic layer is twophotoconductive organic layers and said at least two electrode layers istwo electrode layers.
 24. The device of claim 23 wherein said twophotoconductive organic layers are selected to form a photovoltaicheterojunction.
 25. The device of claim 24 wherein said twophotoconductive organic layers are copper phthalocyanine andperylenetetracarboxylic dianhydride.
 26. The device of claim 24 whereinsaid two photoconductive organic layers are copper phthalocyanine and3,4,9,1 0-perylenetetracarboxylic-bis-benzimidazole.
 27. The device ofclaim 25 wherein said transparent electrode layers comprise indium tinoxide.
 28. The device of claim 26 wherein said transparent electrodelayers comprise indium tin oxide.
 29. The device of claim 1 wherein saidat least one photoconductive organic layer is four photoconductiveorganic layers, having an inner pair and an outer pair and said at leasttwo transparent electrode layers is two transparent electrode layers.30. The device of claim 29 wherein said inner pair of said fourphotoconductive organic layers is a pair of photoconductive organic dyesselected to form a photovoltaic heterojunction and selected to havespectral sensitivity in a specified region of the electromagneticspectrum.
 31. The device of claim 30 wherein said pair ofphotoconductive organic dyes comprises aluminum tris(8-hydroxyquinoline)and 4,4′-bis[N-(1-napthyl)-N-phenyl-amino]biphenyl.
 32. The device ofclaim 30 wherein at least one of said outer pair of said fourphotoconductive organic layers comprises an organic molecular crystalmaterial.
 33. The device of claim 30 wherein at least one of said outerpair of said four photoconductive organic layers comprises a polymericmaterial.
 34. The device of claim 30 wherein at least one of said outerpair of said four photoconductive organic layers comprises a materialselected from the group consisting of phthalocyanine compounds, perylenecompounds, polyacene compounds, and porphyrin compounds.
 35. The deviceof claim 31 wherein each of said outer pair of said four photoconductiveorganic layers comprises a material selected from the group consistingof copper phthalocyanine, perylenetetracarboxylic dianhydride and3,4,9,1 0-perylenetetracarboxylic-bis-benzimidazole.
 36. The device ofclaim 23 wherein said two photoconductive organic layers is a pair oforganic dyes selected to form a photovoltaic heterojunction and selectedto have spectral sensitivity in a specified region of theelectromagnetic spectrum.
 37. The device of claim 36 wherein said twotransparent electrode layers are non-metallic conductive layers andfurther comprising a metallic layer disposed between eachphotoconductive organic dye layer and the adjacent transparent electrodelayer.
 38. The device of claim 2 further comprising a resistive load.39. The device of claim 2 further comprising a power supply.
 40. Thedevice of claim 1 wherein the at least one photoconductive organic layeris a plurality of photoconductive organic layers selected to form aplurality of interfaces for dissociating excitons.
 41. The device ofclaim 40 wherein the thicknesses of said plurality of photoconductiveorganic layers are selected to alter said molecular energy levels ofexcitons so as to form multiple quantum wells in said plurality ofphotoconductive organic layers.
 42. The device of claim 40 wherein saidplurality of photoconductive organic layers are selected from the groupconsisting of organic molecular crystal materials and polymericmaterials.
 43. The device of claim 40 wherein said plurality ofphotoconductive organic layers are selected from the group consisting ofphthalocyanine compounds, perylene compounds, polyacene compounds, andporphyrin compounds.
 44. The device of claim 40 wherein said pluralityof photoconductive organic layers are selected from the group consistingof copper phthalocyanine, perylenetetracarboxylic dianhydride,3,4,9,10-perylenetetracarboxylic-bis-benzimidazole, and vanadylphthalocyanine.
 45. The device of claim 2 wherein said substrate is asubstantially flexible material.
 46. The device of claim 2 wherein saidsubstrate is a substantially rigid material.
 47. The device of claim 2wherein said substrate is a transparent material.
 48. The uevice ofclaim 2 wherein said substrate is a substantially opaque material. 49.The device of claim 2 wherein said substrate is a substantiallyreflective material.
 50. The device of claim 2 as a part of a multipixelphotodetector.
 51. A method of generating electrical power from ambientelectromagnetic radiation comprising: electrically attaching an organicphotovoltaic device having at least two transparent electrode layers toa resistive load; and exposing said device to electromagnetic radiation.52. A method of detecting electromagnetic radiation comprising:electrically attaching an organic photodetector having at least twotransparent electrode layers to a detecting circuit; providingelectrical power to said detecting circuit; exposing said photodetectorto ambient electromagnetic radiation; and receiving electronic signalscorresponding to said ambient electromagnetic radiation from saiddetecting circuit.
 53. An electronic device incorporating the device ofclaim 1, said electronic device selected from the group consisting of aradio, a television, a computer, a calculator, a telephone, a wirelesscommunication device, a watch, an emergency location device, an electricvehicle, an emergency power supply, a power generation device, amonitoring device, an inspection device, a radiation detector, animaging device, and an optical coupling device.